Lecture 6

Look-up-table (LUT)

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Look-up-table (LUT)

Topics for the Next Few Lectures

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3

Program Counter

• Program Counter (PC) is 13-bit wide

• The low byte is the PCL register, which is a

readable and writable register

• The high byte of the PC (PC<12:8>) is not

directly readable nor writable

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directly readable nor writable

– It comes from the PCLATH (PC latch high) register

– PCLATCH holds PC<12:8>

– When PC is loaded with a new value. The contents of

PCLATH are transferred to the upper byte PC

– This occurs during a CALL, GOTO or a write to PCL

PCH and PCL

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CALL and GOTO

• PIC16F84 has 1K of program memory

• CALL and GOTO instructions have an 11-bit address range

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• 11-bit address range allows a branch within a 2K program memory page size

CALL and GOTO

• For future PIC16F8X program memory expansion,

another two bits are required to specify the program

memory page

• These paging bits come from the PCLATH<4:3> bits

• For future memory expansion, the user must ensure that

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• For future memory expansion, the user must ensure that

these page bits are programmed to the desired program

memory page

• For now, PIC16F8X ignores the PCLATH<4:3> bits

• The 2 bits are important for PIC having 4K

(PIC16F873/874 or PIC16F73/74) and 8K

(PIC16F876/877 or PIC16F76/77) program memory

Computed Jump

• You can add W to PCL to make the next program line to jump to somewhere

addwf PCL,1

• This is a computed jump where an offset is added to the program counter

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to the program counter

• But since PCL is only 8 bits, care should be exercised if the jump crosses a PCL memory boundary (each 256 word block)

• If the jump crosses a 00-FF page boundary, you need to set the PCLATH first

movwf PCLATH

Computed GOTO Example - LUT

• It is important to be able to implement look up

tables within microcontrollers

• A common example is in the use of a seven

segment display

• The required values to illuminate the seven

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• The required values to illuminate the seven

segment display are stored in memory in a table

and the correct value extracted when required.

LUT for 7 Segment LED

• The seven segment display is connected to Port

B of the 16F84 as shown below.

rb0

16F84

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rb0

rb1

rb2

rb3

rb4

rb5

rb6

rb7

a

b

c

d

e

f

g

dp

a

b

cd

e

fg

Exercise

• Write the Port B values to display 0 - 9 in the

seven segment LEDs (assuming common anode):

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rb0

rb1

rb2

rb3

rb4

rb5

rb6

rb7

a

b

c

d

e

f

g

dp

a

b

cd

e

fg

16F84

LUT Example

• The LUT will be stored in the program memory

• Generally a value is extracted from a LUT using:

– A pointer - points to the start of the look up table

– An index or offset - this is added to the pointer to extract the

correct value from the table.

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• For example, if the LUT is stored in program memory

starting from address 0x0500x050 c0

0x051 f9

0x052 a4

0x053 b0

.

.

LUT Example

• To display 3 in the LEDs,

– First set up the pointer to point to the start of the table i.e.

0x050.

– Then set up the index 0x03

• PIC microcontrollers handles LUT in an unusual manner

in that the LUT is called up as a subroutine

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in that the LUT is called up as a subroutine

• The pointer is the program counter itself

• The index is a value in the w register which is added to

the program counter

• The value is extracted using retlw - return from

subroutine with literal value in w

LUT Programpcl equ 0x02

portb equ 0x06

movlw 0x03 ;set index

call getval ;call look up table

movlw portb ;output value

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getval addwf pcl,1

retlw 0xc0

retlw 0xf9

retlw 0xa4

retlw 0xb0

.

.

.

LUT

• If getval is located at 0x050 then this will be the

value of the program counter upon entry to the

subroutine.

• The value in w (the index value) is then added to

the program counter

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the program counter

• The program counter now contains 0x054.

– (pc + 1 + index).

– The additional 1 is added because of the pipelining

fetch/execute cycle).

LUT

• The next instruction executed will therefore be

retlw 0xb0.

– i.e. return from subroutine with the value 0xb0 in w.

– Program execution is now directed back to the main

program to the instruction immediately following the

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program to the instruction immediately following the

call to subroutine. i.e. movlw portb.

– The value in w (3) is displayed.

Exercise

• Write a program to find the value of y=2x2 using

a LUT. The possible values of x are 0, 1, 2, 3, 4

and 5.

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8-Level Stack

• PIC16FXX has an 8 deep x 13-bit wide hardware

stack

• The stack space is not part of either program or

data space

• Stack pointer is not readable or writable

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• Stack pointer is not readable or writable

• There are no instructions for pushing or popping

the stack

– They are actions that occur from the execution of the

CALL, RETURN, RETLW and RETFIE instructions

8-Level Stack

• The entire 13-bit PC is “pushed” onto the stack

when a CALL instruction is executed or an

interrupt is acknowledged.

• The stack is “popped” in the event of a

RETURN, RETLW or a RETFIE instruction

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RETURN, RETLW or a RETFIE instruction

execution

• The stack operates as a circular buffer.

– After the stack has been pushed eight times, the ninth

push overwrites the value that was stored from the

first push

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Indirect Addressing

• Allow you to look down a set of files and either

read them or write to them, with the least number

of instructions

• Two files are used for the indirect addressing function.

– File 00h (INDF - INDIRECT File)

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– File 00h (INDF - INDIRECT File)

– File 004h (FSR - File Select Register)

• INDF register is not a physical register.

• Addressing INDF actually addresses the register

whose address is contained in the FSR register

(FSR is a pointer)

Example 1 - Indirect Addressing

• Register file 05 contains the value 10h

• Register file 06 contains the value 0Ah

• Load the value 05 into the FSR register

• A read of the INDF register will return the value

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of 10h

• Increment the value of the FSR register by one

(FSR = 06)

• A read of the INDF register now will return the

value of 0Ah.

Example 2 – Indirect Addressing

• If you want to load 08 into file 1A using Indirect

Addressing Mode:

1. Move 1A into FSR

2. Move 08 into INDF

• 08 gets placed into the file looked at by FSR.

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• 08 gets placed into the file looked at by FSR.

Thus 08 gets loaded into file 1A.

• If we read INDF, we read the contents of the file

looked at by FSR. In other words we read the

value in file 1A and the value is 08!

Exercise

• Write a program to clear RAM locations 20h-2Fh using indirect addressing

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I/O Ports

• Before using any of I/O ports, they must first be

configured as inputs or outputs

• To make a port an input the corresponding TRIS

register should be filled with all ones

• To make a port an output the corresponding

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• To make a port an output the corresponding

TRIS register should be filled with all zeros

Programming I/O Ports

• To access the TRIS registers, register page 1 must be selected.

• Setting bit 5 of the STATUS register selects register page 1.

bsf status, 5

• Clearing bit 5 of the status register selects register page 0.

bcf status,5

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Successive Operations on I/O Ports

• For writing, the actual write to an I/O port

happens at the end of an instruction cycle

• For reading, the data must be valid at the

beginning of the instruction cycle

• Care must be exercised if a write followed by a

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• Care must be exercised if a write followed by a

read operation is carried out on the same I/O

port.

Example

Program to illuminate an LED connected to RB3

+5V

R

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RB3

16F84

Examplelist p=16F84

status equ 0x03

portb equ 0x06

trisb equ 0x06

rp0 equ 5

org 0x00

bsf status,rp0 ;Select reg page 1

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bsf status,rp0 ;Select reg page 1

movlw 0x00 ;Set port b to all

movwf trisb ;outputs

bcf status,rp0 ;Select reg page 0

movlw 0xff ;Turn off all

movwf portb ;LEDs

bcf portb,3 ;Turn on LED

stop goto stop ;

end

Exercise

• Write a program to make all the LEDs connected

to Port B blink three times and then stay on.

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Successive Operations on I/O Ports

• The sequence of instructions should be such that

the pin voltage stabilizes before the next

instruction which causes that file to be read into

the CPU is executed

• Otherwise, the previous state of that pin may be

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• Otherwise, the previous state of that pin may be

read into the CPU rather than the new state

• When in doubt, it is better to separate these

instructions with a NOP or another instruction

not accessing this I/O port.

Bi-Directional I/O Ports

• Any instruction which writes, operates internally as a

read followed by a write operation.

• The BCF and BSF instructions, for example, read the

register into the CPU, execute the bit operation and

write the result back to the register.

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• Caution must be used when these instructions are

applied to a port with both inputs and outputs defined.

• For example, a BSF operation on bit5 of PORTB will

cause all eight bits of PORTB to be read into the CPU.

• Then the BSF operation takes place on bit5 and PORTB

is written to the output latches.

Bi-Directional I/O Ports

• If another bit of PORTB is used as a bi-

directional I/O pin (i.e., bit0) and it is defined as

an input at this time, the input signal present on

the pin itself would be read into the CPU and

rewritten to the data latch of this particular pin,

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rewritten to the data latch of this particular pin,

overwriting the previous content.

• As long as the pin stays in the input mode, no

problem occurs.

• However, if bit0 is switched into output mode

later on, the content of the data latch is unknown.