1. 8254 – SOFTWARE PROGRAMMABLE TIMER

2. Control Word Format

3. 8253/54 Operation Modes Mode 0: Interrupt on terminal count
The output is initially low, and remain low for the duration of the count if GATE=1. When the terminal count is reached, the output will go high and remain high until a new control word or new count number is loaded
If GATE becomes low at the middle of the count, the count will stop and the output will be low. The count resumes when the GATE becomes high again 􀃎 This in effect adds to the total time the output is low.

4. MODE 0: INTERRUPT ON TERMINAL COUNT

5. Modes of Operation
six modes (0–5) of available to each of the 8254 counters
each mode functions with the CLK input, the gate (G) control signal, and OUT signal
Figure 11–35  The six modes of operation for the programmable interval timer. The G input stops the count when 0 in modes 2, 3, and 4.

6. 8253/54 Operation Modes • Mode 1: HW triggered / programmable one shot
The triggering must be done through the GATE input by
sending a 0-to-1 pulse to it.
Steps:
Load the count register
A 0-to-1 pulse must be sent to the GATE input to trigger the count.
After sending the 0-to-1 pulse to GATE, OUT becomes low and stays low for a duration of N*T, then becomes high and stays high until the GATE is triggered again
If during the activation, a retriggered happened, then restart the down counting

7. MODE 1: HARDWARE RETRIGGERABLE ONE-SHOT

8. 8253/54 Operation Modes Mode 2: Rate Generator (Divide-by-N counter)
In Mode2, if GATE=1, OUT will be high for N*T,goes low only for one clock pulse, then counter isreloaded automatically, and the process continues indefinitely. 􀃎 Whole period: (N+1) * T

9. MODE 2: RATE GENERATOR

10. 8253/54 Operation Modes Mode 3: Square wave rate generator
Most commonly used

11. MODE 3: SQUARE WAVE MODe

12. 8253/54 Operation Modes Mode 4: Software triggered strobe
Similar to Mode2, except that the counter is not reloaded automatically
In Mode4, if GATE=1, the output will go high when loading the count, it will stay high for duration N*T.
After the count reaches zero, it becomes low for one clock pulse, then goes high again and stays high until a new command word or new count is loaded
To repeat the strobe, the count must be reloaded

13. MODE 4: SOFTWARE TRIGGERED STROBE

14. 8253/54 Operation Modes Mode 5: Hardware triggered strobe
Similar to Mode4, except that the triggering must be done with the GATE input
The count starts only when a 0-to-1 pulse is sent to the GATE input
If GATE retriggered during the counting, it will restart the down counting

15. MODE 5: HARDWARE TRIGGERED STROBE (RETRIGGERABLE)

16. Single handshake I/O Double handshake I/O SENDER DATA LINES
PORT DEVICE
RECEIVER
STB
ACK
Double handshake I/O
SENDER
RECEIVER
DATA LINES
STB
ACK
PORT DEVICE
STB#
ACK#

17. 8255 PPI - Programmable Peripheral Interface

19. Port Hardware

26. 8251 UNIVERSAL SYNCHRONOUS SYNCHRONOUS RECEIVER TRANSMITTER (USART)

27. There are two types of control word.
Table 1 Operation between a CPU and 8251
Control Words
There are two types of control word.
1. Mode instruction (setting of function)
2. Command (setting of operation)

28. CONTROL WORD(ASYNC. MODE)

29. CONTROL WORD(SYNC. MODE)

30. COMMAND WORD

31. STATUS WORD

32. DTE DCE INTERFACE USING 8251
MODEM
MODEM
TXD
TERMINAL 2
TERMINAL 1
TXD
DTR
DTR
DSR
DSR
TELEPHONE LINE
RTS
RTS CD CD
CTS
CTS
RXD
RXD

33. Typical Printer Cable Centronics male DB25P (male) 36 pins
Connects to PC
Centronics male
36 pins
Connects to printer

36. Port Addresses

37. Printer Interfacing

39. Timing waveform for printer interfacing

40. POP DS CODE SEGMENT ASSUME CS:CODE POP DI START:PUSH CS POP SI POP DS
CLI
MOV AH,25H ;TO MODIFY THE CONTENT OF IVT
MOV AL,00H
LEA DX,TSR
INT 21H
STI ;SET INTURRUPT FLAG
MOV AH,31H ;TO TERMINATE TSR PROGRAM
TSR PROC FAR
MOV AX,CODE ;BCOZ WE DECLARE MSG IN CODE SEGMENT
MOV DS,AX
PUSH AX
PUSH BX
PUSH CX
PUSH DX
PUSH BP
PUSH SP
PUSH SI
PUSH DI
PUSH DS
LEA DX,MSG
MOV AH,09H
POP DS
POP DI
POP SI
POP SP
POP BP
POP DX
POP CX
POP BX
POP AX
MOV AH,4CH ;TERMINATION OF ISR ONLY NOT TSR
INT 21H
IRET
TSR ENDP
MSG DB 10,13,"DIVISION ERROR$"
CODE ENDS
END START

41. CODE SEGMENT
ASSUME CS:CODE
START:MOV AL,12H
MOV DL,00H
DIV DL
MOV AH,4CH
INT 21H
CODE ENDS
END START