1. 8254 SOFTWARE PROGRAMMABLE TIMER/COUNTER

2. Difference between 8253 and 8254 82538254 1Maximum input clock frequency is 2.6 MHz Maximum input clock frequency is 8 MHz 2It does not have read-back featureIt has read-back feature

3. Read back command This command allows the user to check the count value, programmed Mode, and current states of the OUT pin

5. Initializing an 8254 programmable peripheral device Steps:- Determine the system base address for the device. Determine the internal addresses for each of the control registers, ports, timers, status registers, etc., A1A0SELECTS 00COUNTER 0 01COUNTER 1 10COUNTER 2 11CONTROL WORD REGISTER

6. Add each of the internal addresses to the system base address to determine the system address of each of the parts of the device. SYSTEM ADDRESS8254 PART F F 0 1COUNTER 0 F F 0 3COUNTER 1 F F 0 5COUNTER 2 F F 0 7COUNTER REGISTER

7. control word

9. 8254 counter modes and applications

10. Note The following conventions apply to all mode timing diagrams: 1. Counters are programmed for binary (not BCD) counting and for reading/writing least significant byte (LSB) only. 2. The counter is always selected (CS always low). 3. CW stands for ``Control Word''; CW = 10 means a control word of 10 HEX is written to the counter. 4. LSB stands for ``Least Significant Byte'' of count. 5. Numbers below diagrams are count values. The lower number is the least significant byte. The upper number is the most significant byte. Since the counter is programmed to read/write LSB only, the most significant byte cannot be read. N stands for an undefined count. Vertical lines show transitions between count values.

11. Mode 0-Interrupt on terminal count

12. Mode 0-example

13. Mode 1- Hardware-Retriggerable one-shot

14. Mode 1: example

15. Mode 2- Timed interrupt generator

16. Mode 2- examples

17. Mode 3-square-wave mode

18. Mode 3- examples

19. Mode 4- Software-Triggered strobe

20. Mode 4- Examples

21. Mode 5- hardware-Triggered strobe

22. Mode 5- Examples