8051
DESCRIPTION
its a 8051 pdf file. its really gud oneTRANSCRIPT
Intel 8051 microcontroller
David Wilson
8051 Architecture & basic programming model
STUDENT VERSION
Department of Electrical EngineeringUniversity of Karlstad
March 1998
David Wilson1/48
Intel 8051 microcontrollerUNIVERSITY OF
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1 Microprocessors & microcontrollers
This course is really about microcontrollers (8051), not about micro-processors such as the PC 8086.
microprocessors IBM PC, general purpose, multi-chips requiredfor all the functions.
microcontrollers true computer on a chip, logic functions, bit ad-dressing, small cheap & fast. True “computer on a single chip”.
Block diagram of a microprocessor & controller
ALU
AccumulatorWorking registers
prog counter
clock
stack ptr
interrupt circuits
I/O port
I/O port
interrupt
clock
timer
internalROM
ALU
ACCW.Reg.
internalRAM
stack ptr PC
micro-controller
microprocessor
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Microcomputers & microcontrollers
Essential elements of any computer are:
CPU or central processing unit
memory for both data and program
I/O or input/output system
Various different architectures exist depending on the application.
Mini-computer all 3 systems mounted on a board
Micro-computer 2 systems on a single chip.
Micro-controller All systems on a single chip.
The Intel 8051 is an example of a micro-controller.
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2 An micro-controller application
DIW
Measuring house heating
Motivation I have just moved into a villa, and my electricity bill ismuch higher than it was in Australia! So I would like to monitor thetemperature inside and outside of my house in different rooms overat least 48 hours.
Requirement Use one 8051 as a data logger and local storage de-vice. It should measure around 5 temperatures say every minute,and store them so I can load them up from the 8051 via a serial ca-ble to my PC every couple of days.
Heat distribution in my oven & fridge
Motivation I have an old oven without a fan, and I discover thatwhen baking, you must be very careful as to where you place thefood (near the top, or near the bottom etc.) I would like a timevarying 3D temperature map of my oven.
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3 Intel 8051 microcontroller
The 8051 is a Single Chip Computer or microcontroller made byIntel. It is one of the most widely used microcontroller chips in theworld.
� A stand alone, high performance, single chip computer for con-troller applications
� Small, cheap & 40 pins. (# of pins increases size & cost.)
� 64k program memory
� 64k data memory
Our 8031 version is very similar to the generic 8051, but it has somefeatures attractive for proto-typing:
��
��
�
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4 8051 pin layout
The 40 pin package has a pin DIP (dual in-line package) layout asfollows:
But a 44 pin square layout exists and the more advanced membersof the 8051 family have slightly different layouts.
For what is inside this ‘black-box’, see following overheads.
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5 8051 Controller
A minimal 8051-based controller needs only the following compo-nents:
1.
2.
3.
Basic SBC51
Memory8051 CPU
--�
A[0..15]D[0..7]D[0..7]
A[0..15]
- Read#
- WR#
Address bus
-
��
-
input/output
P3.[0..7]P1.[0..7]
� Buses (groups of 8) are wide lines, single lines are bits
� Address (2 buses) give 16 bits (0x0000 – 0xFFFF)
� Data is bidirectional, address unidirectional from CPU to mem-ory
� Read & write strobes one way.
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6 8051 CPU architecture
Compare what crosses the dashed line with the pin-out diagramgiven previously.
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Ref Ayala, p56–57
8051 architecture given previously contains the following:
Registers � 8bit CPU registers (called the ‘A’ register or accumu-lator & B register)
� 16bit program counter & data pointer
� 8bit program status word
� Control registers
Internal ROM of zero (8031) to 4K (8051)
Internal RAM of 128 bytes
� 4 register banks
� 16 bytes (bit addressable)
� 80 bytes of general purpose scratch pad.
Timers & counters (16 bits)
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7 The Single-Board-Computer SBC51
We have a laboratory proto-type single board computer based onthe Intel 8051 called a SBC51. However we will use a PC for editing,storage of source programs and assembling. We will “dump” thecompiled code from the PC to SBC51 using a serial cable.
SBC51 board components
DisplayPort #1
Serial to PC
80C32
EPROM
8255
switches Digital outAnalogue in/out
Singlestep
reset
switch
See the component layout on p58, and the circuit diagram follow-ing. Also read chapter 9 of Ayala.
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A picture of an equivalent single board computer proto-type kit.(Not exactly the same as what we have in the lab)
This configuration has
� 80C32 (square package) with BASIC & external ROM
� liquid crystal display (LCD) screen, 2 by 16
� Serial connection (to dump programs & data)
� hexadecimal keypad (4� 5) buttons
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Getting started with the SBC51 Read this before the first laboratory.
1. Connect the PC port COM1to the SBC port P1 with the serialcable, 9600, no parity & 1 stop bit. Use MSD.EXEto check &mode to check your settings.
2. Turn on the SBC51 & reboot the PC under DOS. (It will givestrange results under Windows or NT, possibly not communi-cating)
3. Choose the SBC51or andra program in the boot menu, andthen start the program:
c:> cd sbc51\work
c:> sbc51\work\sbc.exe
4. Load your program using the LOAD FILE menu.
5. Edit your file if necessary, use ALT-h for help.
6. Assemble your program to a relocatable object file, a R03, thenlink to an Intel hexadecimal HEX file.
7. Choose TERMINALand
(a) press ALT-C to set up the serial communication to theSBC51.
(b) Press the RESET button on the SBC51
(c) Press ALT-L to download the hexcode.
8. Start with the laboratories, p3 1.1
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8 8051 Memory types
The 8051 has two separate memory blocks, for data and program.Since both blocks have the same address, this is called a Harvardarchitecture.
Separating the program memory from the data memory improvesreliablility since we cannot inadventently overwrite the programcode, and allows us to use program ROM, or read only memory.
Program memory normally we will have 4K on the chip (but not inthe case of the 8031)
4K internal
60K externalor 64K external
80318051(our proto-type version)
Program memory
Data memory all 64K is off the chip except for (a miniscule 128bytes).
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Data memory internal 256 bytes is divided into two parts: generalscratch and the special function registers SFRs. (See description fol-lowing).
00
and7F80
128 bytesinternal
Data memory
128 bytesinternal
SFR64K
external
FF
00
FFFF
8><>:
8><>:direct addressing only
direct & indirect addressing
The 128 bytes in lower internal RAM have the following structure:
.................................
.................................
.................................
9>=>;
bank #0bank #1bank #2bank #3
7 6 5 4 3 2 1 0
register banks
.................................
................................. �
bit addressable area
scratch pad
0007
� � � 201F
7F
� Only one of the 4 register banks can be active at any particulartime
� Which one is active is given by flags in the PSW
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9 Special Function Registers
The Special Function Registers (SFRs) contain memory locationsthat are used for special tasks. (They should not be used for gen-eral purpose tasks.)
Each SFR occupies internal RAM from 0x80 to 0xFF, (but some areasare empty!) They are 8 bits wide. Some examples are:
A register or accumulator is used for most ALU operations & exter-nal moves
B used for mltiplication & division and can also be used for generalpurpose storage
PSW Program Status Word is a bit addressable register. Refer to themanual for the description of the bit fields.
See p48
Two special 16-bit registers (double registers):
PC or program counter. This is not directly addressable, nor does ithave a memory location
DPTR or data pointer. Is accessible two 8-bit registers: DPTRLowand DPThigh.
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Program Status word (PSW)
Ayala, p62
The PSW is the most important of the SFRs. It is a byte registerwhich holds 8 bits that can addressed indivdually. Each bit is re-ferred to as a flag.
Flags can be either set (1) or cleared (0). This is more efficient thanusing an entire byte memory location for a binary variable.
The PSW is divided up as follows:7 6 5 4 3 2 1 0
CY AC F0 RS1 RS0 OV – PNote:
� We start numbering from zero and go right to left (MSB, or mostsignificant bit first)
� Flags are zero and one (not 1 & 2)� One flag should not be used, but is reversed for future use. Use
at your own peril
� Address as PSW.3 for RS0 for example.
Each of the flags has a special function, consult the manual.
Question What flags are set when the PSW holds 0x7C ?
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Bit addressable SFRsSome special function registers are bit addressable. (This makes iteasier to program.)
The byte addresses are always multiples of 8, or they all lie in column0 of the SFR register block. The bit addresses are just appended ontothe byte address.
So the fouth bit of the accumulator is Acc.3 .byte address of the accumulator is 0E0h
so, the bit address is:
Acc.3 = 0E0h
+ 5
= 0E5h
Question: How does this explains the (incorrect) command CLR
B?
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10 8051 flags
Flags are stored in the PSW, so will be affected if you change it (ex-cept the parity flag).
Arithmetic flags:
� C, AC & OV, (Carry, auxiliary carry & overflow)
� Increment/decrement do not effect the math flags.
� I.e. no OV if we INC A from FFh to 00h .
� inc DPTR , note no DECversion.
Addition
C if carry out of bit #7
AC is set if carry from bit 3 to bit 4. (Used in BCD addition)
OV is set if carry out of bit 7 but not bit 6oris set if carry out of bit 6 but not bit 7.In other words:
OV = C7XORC6
Signed addition & subtraction is complicated, and you will need tostudy the flags carefully.
Use pre-tested subroutines for your arithmetic routines if possible.(See text books, or libraries on the internet.)
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11 Accessing external memory
Storage problems? What happens if our program is larger than4096 bytes of code?We have the ‘ROMless’ 8031 version, so we must
In assembler we use the movx (move external) to access the datablock, and movc to fetch from code memory.
We have two separate read signals, RD, (read) and PSEN , (pro-gram send enable). The bar means that it is active low, (when zero,will do something).
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12 Prototyping 8051 design
Ayala, Chapter 9
For many small applications where we might want to change theROM code, we would like to use external EPROM, which means:
� We have lost the use of ports 0 and ports 2 since they must beused to interface with external ROM and not available for I/O.
� We may need external RAM (since the 128 bytes may not beenough for our application), which means that we have lostport 3.6, WR, and 3.7, RD.
� Any serial communication requires 3.0 (RXD) and 3.1 (TXD)
Loss of ports leaves us with:
1. All of port #1, and
2. Port P3.2 – P3.5
for general purpose I/O and external interrupts and timing inputs.
Question How do we recover the use of the lost ports?
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Programmable Logic DevicesThe PROM chip is an example of a Programmable Logic DevicePLD. Once programmed, it will retain the program even when thepower is turned off.
� One-time programmable devices uses tiny fuses which areburnt or blown (or we can use antifuses which have the sameend function. Circuit size is 10� that for EPROMS.
� Eraseable Programmable read Only memory (EPROMS) can bere-programmed. This uses a modified MOS transistor with afloating gate that when uncharged does not effect the normaloperation. However if it is subjected to (high) +12V, then acharge will move into the 2nd transistor which is stable (willlast a decade).
Reprogram by subjecting the cells to UV light (via an expensiveceramic window on top of the package). Wiping time takesabout 20 minutes.
� EEPROM or Electrically erasable proms are about 2.5 timeslarger than eproms, and are quicker to clear.
� FLASH proms are much quicker to erase (hence the name), andcan be reprogrammed while still on the circuit board. Suitable foreasy upgrading by the public of ROM chips in Modems andPCs.
On the SBC51 board we have a 32k EPROM that is reprogrammedby UV light. This holds the monitor program. (We don’t have thesource to this, but we could reverse assemble it.)
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13 Reading to Ports
1. Store the address of the memory mapped port into the DPTR
register, and
2. indirectly address this through Acc.
; Reading & writing from a PORT
MOV DPTR, #0F012h; address to read digi-
tal input
MOVX A, @DPTR ; read the val-
ues into Acc
MOV DPTR, #0F011h; address of LED outputs
MOVX @DPTR, A ; write this value back out agai
The actual values of the address are decided by the designed of theparticular circuit board, (SBC in our case).
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14 Subroutines
Question Subroutine overheads slow a program down so why use them?
Subroutines do exist in assembler programming using the CALLop-code, but you must take care of the “housekeeping” yourself.
� All variables are global, so they may get altered or destroyed.This includes the PSW, register banks etc.
� You will need to save all variables before calling, and restorethem after returning from the subroutine.
� The stack is a special memory location (stack of registers) thatyou can use. (see OHs following)
� The processor uses the stack to remember the address where toreturn to after the subroutine has finished.
� The stack is pointed to by the stack pointer, SP.
� Near, Long (or far) and absolute calls are possible.
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Ayala, p64, p129–130
Stack & the stack pointer makes housekeeping easier for certaintasks like storing & retrieving numbers quickly. The name comesfrom a ‘stack’ of dishes in a cafeteria or a stack of cigarettes at thesupermarket checkout.
To store data on the stack you PUSHit on.To retreive data, you POPit off
The Stack is a small amount of internal ram. The start of the stackis held in the stack pointer, SP. As you push data, the stack growsupwards (to higher locations) in ram memory.
The following occurs when you store some data on the stack & thenpop it off again.
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Ayala, p130
Stack Note the cautions regarding usage!
� The stack is reset to 07h, but since this will overwrite the reg-ister banks, and bit area, we normally move it ‘out of harmsway’, to say 3Fh. (Done by the monitor program)
� If you push too many times, you will run off the end of internalRAM adress 07h which will result in errors.
� SP “rolls over” from FFh to 00.
� Use direct address, not register names R1, R2 etc since the stackhas no way of knowing which register bank is in use at thetime.
� Use PUSH Accnot PUSH A.
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15 The stack & subroutine calls
Ayala, p177
What happens when a subroutine is called (including a hardware inter-rupt) ?
1. The PC is incremented and this becomes the return address,i.e. the spot the CPU will return to after it has completed thesubroutine.
2. The 16bit return address is pushed onto the stack, low bytefirst,
3. during which the stack pointer SP is incremented twice.
4. The subrouine address is placed in the PC and the subroutineis executed, until . . .
5. a RETreturn instruction is encounted, then
6. The CPU POPs the stack twice to recover the return address,and places it in the PC. (Sp is decremented twice)
7. CPU starts from the return address . . .
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Question What is the maximum number of nested subroutine callswe can make?Question What are the advantages/disadvantages of long calls as
opposed to short calls?
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16 Interrupts
Interrupts are just special subroutines that may (or may not) becalled explicity.
Why have them?
1.
2.
3.
If conditions are “right”, when an interrupt occurs, then the proces-sor will stop what it is doing, and jump to a specific place in mem-ory (decided by the Intel 8051 designers) hooked that that particularinterrupt. (See following OH.)
It is up to the programmer to make sure that you supply a sensiblefurther course of action. This is called the interrupt handler routineor interrupt service routine, ISR.
Types of interruptsOn the 8051 we have 2 timers or counters and 2 external interrupts.
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Interrupt programming
� Signals or conditions generated external to the main program
– External events such as a change in a logic value
– Overflow of a counter
– Arrival of data at the serial port.
� Interrupts can be enabled or disabled by the programmer. (Al-though some interrupts are non-maskable.)
Interrupts on the 8051
� IE0 - external interrupt #0. Provided by an input pin on thechip.
� IE1 - external interrupt #1. Provided by an input pin on thechip.
� TF0, TF1 - Timer/Counter Interrupts. Generated by internaltimer/counters
� RI,TI Communication port interrupts. Indicates that a charac-ter has been received or the buffer is empty and a character cnabe transmitted.
Each interrupt forces the processor to jump to a different place inmemory.
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Note that the computer does not explicitly call the ISR, this is de-termined by outside events, & we cannot predict exactly where &when it will call.
This splits out program into 2 parts, foreground & backgroundtasks.
Foreground tasks are under program control & are called explicitly
Background tasks are called via interrupts.
This makes the writing of complex real-time controller applicationsmuch easier.
Relocating the ISRs Normally ROM (or EPROM) occupies lowmemory, so the ISR is relocated to a higher address. (Otherwisewe would need to re-program the ROM everytime we wanted tochange the ISR in our prototype.)The Interrupt vector tableThe interrupt table has been moved (by the monitor program).
Name 8051 relocated on the SBC51INT 0 0003 single-step functionTIMER 0 000B 200BINT 1 0013 2013TIMER 1 001B 201Bserial port 0023 2023
Usage is by the LCALL op-code. However you must supply a RETI ,a return from interrupt op-code at the end of your interrupt handlersubroutine.
In our case we do a Long jump to +2000h.
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17 Timers & counters
Timers (clocks) and counters are a good example for the advantagesof interrupts. Many applications require the counting of externalevents;
� Calculate the frequency of an external pulse train, or
� generate a precise delay between other computer actions (sayfor digital control)
Could use software techniques, but this keeps the processor occu-pied. Better to use interrupts & the two 16-bit count-up timers. Caneither be programmed to:
1. count internal : : :
2. count external : : :
All counter action is controlled by the TMOD(timer mode register)and the TCON(timer/counter control register).
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TCON Timer control SFR contains timer 1&2 overflow flags, exter-nal interrupt flags, timer control bits, falling edge/Low level selec-tor bit etc. (refer documentation)
TMOD timer mode SFR is two four-bit registers (timer #1, timer #0).Select timer/counter & various modes.)
Applications Say we want to count a specified number of events(clock pulses or external events), then
1. Store the start number in the counter. (Value = max count- de-sired count+1)
2. Counter automatically increments (in the background)
3. When it rolls over to zero, it will ste the timer flag.
4. Test the flag in the program, or generate an interrupt.
Timing configures the counter to count the internal clock fre-quency/12. (E.g. if fc = 6:0Mhz, then the timer clock will havea frequency of 500kHz.)Configure as a timer:
1. Clear C= �T bit in TMOD(Count internal frequency)
2. Set TRx in the TCON(timer run) and the gate bit in the TMOD
must be 0, or the external pin �INTx must be 1.
3. Select one of 4 modes.
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Ref: Ayala, pp205–212
Timing subroutinesIn microcontroller applications we need to wait a specified time,and so we need to have programmable time delays.
In all cases your application will be written for a specific clock fre-quency, say 12MHz, 16MHz, 11.0592 MHz etc.)
If you subsequently change the clock frequency, you will need toupdate your code, & possibly do some timing tests.
The timing options are:
1. Pure software time delays (wastefull)
2. Software polled timer (still wastefull, but OK in non-criticalapps)
3. Pure hardware using interrupts (accurate & preferred method)
Question We are mostly interested in the hardware timing version,why?
You should read carefully the Pure Hardware Delay code exampleon pp210–212 in Ayala.
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Ref: Ayala, p210
Using a timer interrupt allows a timer to run and generate a timerinterrupt while the main program is running.
1. Setup the timer for a delay of 1000 �s. Enable the interrupts,and place the interrupt vector at 001Bh .
2.
3.
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18 Computer communications
Phil Melore, http://www.plcs.net/comhistory29.htm
Computer communication systems have their roots in the old telegraph. Early attempts
to communicate electronically over long distances began as early as the late 1700’s. In
1810 a German man (von Soemmering) was using a device with 26 wires (1 for each letter
of the alphabet) attached to the bottom of an aquarium. When current passed through
the wires, electrolytic action produced small bubbles. By choosing the appropriate wires
to energize, he was able to send encoded messages “via bubbles”. This then caught the
attention of the military and the race to find a system was on.
In 1839, 2 Englishmen, Cooke andWheatstone, had a 13 mile telegraph inuse by a British railroad. Their devicehad 5 wires powering small electromag-nets which deflected low-mass needles.By applying current to different combi-nations of 2 wires at a time the needleswere deflected so that they pointed toletters of the alphabet arranged in a ma-trix. This “2 of 5” code only allowed20 combinations so the letters “z,v,u,q,jand c” were omitted (forget about a, a,and o). This telegraph was a big step forthe time, but the code was not binary(on/off) but rather it was trinary (theneedle moved left,right,or not at all).
The biggest problems with these devices was the fact that they were parallel (required
multiple wires) so it was S.F.B. Morse who created the fully serial binary system; Morse
code.
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19 Interfacing
Usually means connecting your PC to periperals such as printers,plotters, modems, data acquisition hardware etc.
My portable PC
serialparallel????
????????
8-bit
| {z }
My PC LP1:
COM1:
1 bit + control
For interfacing we have two options
Parallel all 8 (or more) bits used together to send an entire byteat a time. (Requires about 20 pins practically) Speed is about150kbits/second for a maximum distance of 2–3 meters. (Nostandard, although CENTRONIX PARALLEL, or ECP, or EPP (ex-tended parallel port) are common. Writes one byte at a time,reads one nybble at a time.
Serial Send just one bit at a time. (Requires 5–8 pins practically.)Follows RS-232 standard and extensions. Typical speed is 9600bits/second (although faster versions are more common nowwith compression etc.)
Easy to program:Use the IN and OUTop-codes on the 8086 PC. or use the Port[]
command in Pascal.
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20 Serial communications: RS232
The serial RS232 standard is a serial asynchronous communicationsprotocol. (RS is for ‘recommended standard’)
Serial The infomation travels down a single line
Asynchronous means that it is not necessary to exactly synchronisethe clocks between the transmitter and receiver. Data is trans-mitted from 1 computer to another, not necessarily running atexactly at the same clock speed.
Standard An Electronics Industry Standard (EIS) #232. Defineshow the connectors should physically look, specifies voltagelevels, and defines what signals should do.
There are 2 types of RS-232 devices.
DTE Data Terminal Equipment and a common example is a com-puter. (Originally PCs used to be thought of as terminals to realcomputers such as IBM mainframes)
DCE Data Communications Equipment and a common example isa mainframe or modem. The 8051 may be either a DTE or DCEdevice.
and there are now two types of connectors:
1. The older 25 pin D-shell (DB–25)
2. The newer 9pin (DB–9), since most of the connections on the 25pin version were never used.
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Serial port hardware comes in 2 flavors– a 25-pin type and a 9-pintype (shown below).
9-PIN 25-PIN PURPOSE1 1 frame ground (internally connected to the chassis)2 3 receive data (RD) (pin where the data from the external device enters)3 2 transmit data (TD) (pin where the data leaves)4 20 data terminal ready (DTR)5 7 signal ground6 6 data set ready (DSR)7 4 request to send (RTS)8 5 clear to send (CTS)9 22 ring indicator (RI) *only for modems*
Details of the pins follows:
DTS data terminal ready– This pin is a master control for the ex-ternal device. When this pin is 1 the external device will nottransmit or receive data.
DSR data set ready– Usually external devices have this pin as apermanent 0 and the plc basically uses it to determine that theexternal device is powered up and ready.
RTS request to send– This is part of hardware handshaking. Whenthe 8051 wants to send data to the external device it sets thispin to a 0. In other words, it sets the pin to a 0 and basicallysays “I want to send you data. Is it ok?” The external devicesays it’s OK to send data by setting its clear to send pin to 0.The 8051 then sends the data.
CTS clear to send- This is the other half of hardware handshaking.As noted above, the external device sets this pin to 0 when it isready to receive data from the 8051.
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Ref: Sargent & Shoemaker, p655
The DB 9pin male conector as seen looking at the computer’s serialport. This is smaller & has less pins than the 25pin version, but stillcarries all the necessary infomation.
1
2
3
4
5
7
6
9
8
request to send (RTS)
Data set ready
clear to send (CTS)
Ring indicator (RI)
Data carrier detect (DCD)
Receive data (RxD)
Transmit Data (TxD)
Data terminal ready (DTR)
Signal ground
DB-9 male connector & pin assignments
DB–25 pin connections for RS232 lines.
RS232 signal Direction DTE DCESignal ground 1 1Transmit Data (TxD) Out 2 3Receive Data (RxD) In 3 2Request to Send (RTS) Out 4 5Clear to Send (CTS) In 5 4Data Terminal Ready (DTR) Out 20 6Data Set Ready (DSR) In 6 20Ring Indicator (RI) 22 22
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Connecting two DCE devices (such as say two computers) will notwork very well, (since they are both DCEs, rather than one DCE andone DTE) so we must reverse two of the pins.
2 receive data
3 transmit data
2 receive data
DTE device DTE device
3 transmit data
2 receive data
3 transmit data
2 receive data
DTE device DTE device
3 transmit data
Null modem cable
Will not work !
computer #2computer #1
It is easy to swap over 2 of the lines of the cable to make a NullModem Cable.
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Serial RS232 protocolSo to transmit the letter ‘A’, ASCII code (41h), we must send
‘A’ � 01000001
where:1 = high voltage, ( > +5V), but less than +15V0 = low voltage, ( < �5V ), but greater than �15V
So the serial transmission of ‘A’ is
Serial
1 0 0 0 0 0 01
�direction of time
z }| {‘A’
Without any transmission is kept at a high voltage.
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But this brings in a ambiguity regarding the start of the transmis-sion. (We could tune in half-way through.)
What happens if the transmission starts with a 1?
Start bit Add a zero to the front of each character transmitted. (Thisis obviously not part of the information content of the signal.) Sonow ‘A’ is sent as: 0 01000001| {z }
A
.
Noise in telephone lines is very common.
� Data can get corrupted by random signals
� Need a redundancy check. (Will not be fool proof, but reducesthe probability of a mistaken signal)
� This will further lengthen the code, and reduce the ‘informa-tion concentration’
Odd Parity. A cheap redundancy check. Count how many 1s wehave in our character, and if the number of ones is:
Even, then add a one to the codeOdd, then add a zero to the code
This ensures that each transmission segement (character) has an oddnumber of ones. Even parity is possible, and in this case all charac-ters have an even # of ones.
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Using Odd parity, to transmit the character B
01000010| {z }
2ones
which has an even number of ones, we must append after the lastbit a single 1 to get:
parityz}|{1 01000010| {z }
B
Now to transmit C=
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21 Interfacing with the outside world
For the computer to measure ‘real-world’ signals such as tempera-ture, pressure, humidity, and for it to write out voltages to controlthe speed on motors, brightness of lights etc, it must have both a
� D/A (Digital to analogue converter)
� A/D (Analogue to digital converter)
-voltage in
9=; 8 bit (1 byte) to computerA/D
-
8-bit Analogue to Digital conversion
--------
8<:8 bit binary value D/A - voltage out
8-bit Digital to Analogue conversion
-------
More expensive 12 & 16 bit converters are also possible.
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D/A converters are easier to fabricate and cheaper than A/Ds. TheD/A takes a binary stream, and converts this to a single continuousvoltage.
D/A converter
R2R4R8R
b0 b1 b2 b3
�Vref
Vout
R0
�+
The D/A converter outputs a voltage, Vout given a binary number
b3b2b1b0, where each of the bs can be either 1 or 0,
Vout =R0
R
b3 +
b22
+
b14
+
b08
!Vref
As we increase the # of bits, the range of matched resistors becomeslarge, and the accuracy becomes poor.
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Ref: Ogata, DCE, p32
An alternative scheme for a D/A converter that uses resistors ofonly size R and 2R is a ladder scheme.
Ladder D/A converter
2RRR
2R2R
b1 b2 b3
�Vref
R2R
2R2R
b0
Vout
3R
�+
In this scheme, all the resistors (except for the feedback resistor)are either R or 2R which means a high level of accuracy can beachieved.
A/D converters are more complicated and use internally a D/Aconverter coupled with a successive approximation register (SAR).Details given in electronic handbooks.
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22 8052: Extending the 8051 family
The 8051 is just the first member of a series of micro-controllers. Themost common “big brother” is the 8052.
� adds another 128 bytes of internal RAM (indirectly addressable)
� increases the onboard ROM from 4K to 8K.
� Adds another timer, T2, (to make three; T0,T1 and T2), andassociated new SFRs.
� T2 is a capture timer.
� No new op-codes, but does have new SFRs
Since the INTEL designers of the original 8051 left ample room inthe SFR space for future functions, Philips Semiconductor designerscan add another timer with special functions, without adding new op-codes.SFR AddressT2H 0xCD
T2L 0xCC
RCAP2H 0xCB
RCAP2L 0xCA
T2CON 0xC8
You should consult the relevant manufacturer’s documentation forthese extra functions, (such as Phillips Semiconductor).
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23 8051 conclusions
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