ubc104 embedded systems review: interrupts & timers

UBC104 Embedded Systems

Review: Interrupts & Timers

UBC 104 Embedded Systems 2

Block Diagram


Interrupts

Definition of ‘Interrupt’

Event that disrupts the normal execution of a program and causes the execution of special

instructions


Interrupt Handling Code that deals with interrupts:

Interrupt Handler or Interrupt Service Routines (ISRs)

Possible code:

void ISR(void) interrupt 1 {++interruptcnt;

}

Interrupt number


Interrupt Overheads Interrupt arrives

Complete current instruction

Save essential register information

Vector to ISR

Save additional register information

Execute body of ISR

Restore other register information

Return from interrupt and restore essential

registers

Resume task

InterruptLatency

InterruptTermination


SFR Map – Timer


Timer Codevoid TimerInit(void) { T2CON = 0x04; // Load Timer 2 control register

TH2 = 0xFC; // Load Timer 2 high byte RCAP2H = 0xFC; // Load Timer 2 reload capt. reg.

high byte TL2 = 0x18; // Load Timer 2 low byte RCAP2L = 0x18; // Load Timer 2 reload capt. reg. low

byte

ET2 = 1; // Enable interrupt TR2 = 1; // Start Timer 2 running}

void handleTimer2 (void) interrupt 5 {/* execute interrupt code */

}

Initialization

Start of Timer

Interrupt Service Routine


Calculation of Register Settings

Oscillator = 24 MHz Timer clock= (24 / 12) MHz = 2 MHz Timer cycle = = 500ns Delay count = delay time / cycle time

1 ms / 500 ns = 2000 Base number= 213-2000 = 8192-2000 = 6192 = 0x1830

2 MHz1


Summary: Interrupts & Timers Interrupts:

Event that disrupts the normal execution of a program and causes the execution of special instructions

Interrupt Latency: Time from event to execution of service routine

Interrupt Response Time:Interrupt latency + Time for service routine

Interrupt Termination: Time taken after interrupt service routine

Timer: Counter that causes interrupt at overflow

UBC104 Embedded Systems

RS232 Communication


Overview

Basics of serial communications RS232 details 8051-support for serial communication


SFR Map – UART Registers


UART Registers


Serial Interface Basics Also called

Universal Asynchronus

Receiver/ Transmitter (UART)

or after the I standards: RS232 (-C) or EIA232


Serial Interface Basics

also called Universal Asynchronus Receiver/ Transmitter (UART)

or the relevant standards: RS232 (-C) or EIA232

2 Receive Data

3 Transmit Data

5 Signal Ground


Typical 3-wire Interface

3

2

2

3

5

5


uController Pin-Out


RS232 Signals

Signals between +25V and -25V;

some say ±15V

usually +12V to -12V


RS232 Signals


some say ±15V

usually +12V to -12V Logic ‘1’

Logic ‘0’


RS232 Signals


some say ±15V

usually +12V to -12V


RS232 Signals


some say ±15V

usually +12V to -12V 8051 runs on 3V or 5V


RS232 Signals


some say ±15V

usually +12V to -12V 8051 runs on 3V or 5V

Driver chip translates between voltages


Valid Signals


Valid Signals

Figure courtesy of http://www.camiresearch.com


MCBx51 Board

TX0RX0

23


Basic 3-Wire Connection of Machines

TXD

GND

RXD


Basic 3-Wire Connection of Machines

What goes over these wires?

TXD

GND

RXD


RS-232 Frame

Every RS-232 consists of: 1 start bit 8 data bits 1 stop bit (optional 1 parity bit)


RS-232 Frame

Start D0 D1 D2 D3 D4 D5 D6 D7 Stop



RS-232 Frame



‘a’= 0x61 = 0110 0001


RS-232 Frame


1 1 1 1


‘a’= 0x61 = 0110 0001

1

0


RS-232 Frame


1 1 1 1 0


‘a’= 0x61 = 0110 0001

1

0


RS-232 Frame


1 1 1 1 0 1 0 0 0 0 1 1 0


‘a’= 0x61 = 0110 0001

1

0


RS-232 Frame


1 1 1 1 0 1 0 0 0 0 1 1 0 0


‘a’= 0x61 = 0110 0001

1

0


RS-232 Frame


1 1 1 1 0 1 0 0 0 0 1 1 0 0 1 1


‘a’= 0x61 = 0110 0001

1

0


Signal Timing

1 1 1 1 0 1 0 0 0 0 1 1 0 0 1 1 1 1


?


Signal Timing (continued)

1 1 1 1 0 1 0 0 0 0 1 1 0 0 1 1 1 1


?bit period


Baud Rate

Baud specifies the inverse of the bit-period

e.g. 9600 Baud = a bit-period of 1/9600 second

= 104.2 microseconds

Typicall data rates: 75, 110, 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 33600, 56000, 115000 and (rarely) 330000 baud.


Serial I/O with bare hands#define BIT_PERIOD 1042

sendByte(unsigned char content) {unsigned char mask;

mask= 1;P3.1= 0; // Start bit

wait (BIT_PERIOD);

for (i= 0; i<8; i++) { // Send contentP3.1= (content&mask) ? 1 : 0;

wait (BIT_PERIOD);mask<= 1;

}

P3.1= 0; // Stop bit wait (BIT_PERIOD);

P3.1= 1;}





wait (BIT_PERIOD);



}


P3.1= 1;}





wait (BIT_PERIOD);



}


P3.1= 1;}

You do not need to do this!!!


UART modes Mode 0: 8 data bits (LSB first) are transmitted/received at a fixed baud rate of

1/12 of the oscillator frequency. (synchronus mode)



1/12 of the oscillator frequency. (synchronus mode) Mode 1: 10 bits are transmitted (through TXD) or received (through RXD): a

start bit (0), 8 data bits (LSB first), and a stop bit (1) at a variable baud rate.




start bit (0), 8 data bits (LSB first), and a stop bit (1) at a variable baud rate. Mode 2: 11 bits are transmitted (through TXD) or received (through RXD): a

start bit (0), 8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On transmit, the 9th data bit (TB8 in SCON) can be assigned the value of 0 or 1 (for example, the parity bit (P, in the PSW) could be moved into TB8). On receive, the 9th data bit goes into RB8 in Special Function register SCON. The baud rate is programmable to either 1/32 or 1/64 the oscillator frequency.

Mode 3: Mode 3 is the same as Mode 2 in all respects except the baud rate is variable.




start bit (0), 8 data bits (LSB first), and a stop bit (1) at a variable baud rate. Mode 2: 11 bits are transmitted (through TXD) or received (through RXD): a

start bit (0), 8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On transmit, the 9th data bit (TB8 in SCON) can be assigned the value of 0 or 1 (for example, the parity bit (P, in the PSW) could be moved into TB8). On receive, the 9th data bit goes into RB8 in Special Function register SCON. The baud rate is programmable to either 1/32 or 1/64 the oscillator frequency.

Mode 3: Mode 3 is the same as Mode 2 in all respects except the baud rate is variable.

Mode 0 Baud Rate = 12

Oscillator Frequency

Mode 2 Baud Rate = 2 SMOD1 x 64


Mode 1/3 Baud Rate = 2 SMOD1 x 32

Timer Overflow Rate


SMOD1

SMOD1: Double Baud rate SMOD0: Enables framing error detection


Framing Error Detection


Timer 1

http://www.keil.com/c51/baudrate.asp

Mode 1/3 Baud Rate = 2 SMOD1 x 32

Timer Overflow Rate

Mode 1/3 Baud Rate = 12 x [256 – TH1]

Oscillator Frequency2 SMOD1

x32

Baud Rate[256 – TH1] =


x32 12 x

Baud RateTH1 = 256 –


x32 12 x


Timer 1 Commonly Used BAUD Rates


SCON Documentation


SCON & SBUF

SCON controls the functionality of the UART device: SM0&SM1 = Determine the mode SM2 = Multiprocessor Flag REN = Receive Enable Flag TB8/RB8 = Transmit/Receive Parity Flag TI/RI = Transmit/Receive Interrupt Flag

SBUF is the transmit&receive buffer


SBUF

SBUF is actually two separate registers: a transmit buffer and a receive buffer register: When data is moved to SBUF, it goes to the transmit buffer

where it is held for serial transmission; moving a byte to SBUF initiates the transmission.

When data is moved from SBUF, it comes from the receive buffer.


SBUF and TI/RI

8 data

SBUF

8

TI

Stop bit

Start bit

Send

8-bitdata

Transmitter

Buffer isempty

10 bitparallelto serialconversion

Serial data transmit

TRANSMITTER HALF

8 data bits

startbit

stopbit

Tx

bitbit

8 data

SBUF

8

RI

Start bit

Stop bit

Receive

8-bitdata

Receive

data is

available

10 bitserial toparallelconversion

Serial data receive

RECEIVER HALF

8 data bits

stopstart

Rx


SBUF and Parity Bit in TB8/RB8

8 dataSBUF

8

TI

Stop bit

Start bit

Send8-bitdata

TransmitterBuffer is

empty

11 bitparallelto serialconversion

Serial data transmit

8 data bitsstart

bitstop

bit

Tx

9th. bit

TB8Put parity bit here

(ninth bit!)

p

parity bit

8 data

SBUF

8

RI

Start bit

Stop bit

Receive8-bitdata

Receivedata is

available

11 bitserial toparallelconversion

Serial data receive

8 data bits

stopbit

startbit

Rx

9th. bit

p

parity bit

RB8Read parity bit here

(ninth bit!)


Timer 1 Example

1. Set T1 for Mode 2

2. Load TH1 with the initial value

3. Set SCON for Mode 1

4. Start Timer1

5. Clear TI

6. Load SBUF with the byte to be transferred

7. Wait until TI becomes 1

8. Go back to Step5 for next byte

TCON= 0x20;

TH1= 0xF5;

SCON|= 0x50;

TR1= 1;

TI= 0;

do {

SBUF= byte[i];

while (!TI);

} while(more_bytes);


Timer 1 Example (continued)




4. Start Timer1

5. Clear RI

6. Wait for byte to be received

7. Copy received byte out of SBUF

8. Return received byte

TCON= 0x20;

TH1= 0xF5;

SCON|= 0x50;

TR1= 1;

RI= 0;

while (!RI);

result= SBUF;

return SBUF;


Possible Configurations


T2CON Documentation


Timer 2 Commonly Used BAUD Rates


Timer 2 Calculation

Mode 1/3 Baud Rate = 32 x [65536 – (RCAP2H, RCAP2L)]


Baud Rate(RCAP2H,RCAP2L) = 65535 –


32 x


Timer2 Example


2. Load RCAP high/low with the initial value


4. Start Timer1

5. Clear TI




T2CON|= 0x30;

RCAP2H = 0xFF;

RCAP2L = 0xB2;

SCON|= 0x50;

TR2= 1;

TI= 0;

do {

SBUF= byte[i];

while (!TI);



Internal Baud-Rate Generator


BDRCON


Using BRL to Transmit




4. Start Timer1

5. Clear TI




BDRCON= 0x0E;

BRL= 0xF5;

SCON|= 0x50;

BDRCON|= 0x10;

TI= 0;

do {

SBUF= byte[i];

while (!TI);


ubc104 embedded systems review: interrupts & timers

Documents

interrupt overheads

interrupt number slide

interrupt code

interrupt handler

interrupt tr2

eia232 slide

ubc104 embedded systems

overflow slide