Explain with proper diagram all the six modes of operation of programmable interval timer 8254.

Mode 0: The output in mode 0 is initially low, and will stay low for the period of the count if GATE = 1.

Width of low pulse = N×T

Here N is the clock count loaded into counter, and T is the clock period of the CLK input.

If the terminal count is reached, the output will go high and stay in high till a new control word or new count number is loaded in it. In mode 0, if GATE input becomes low at the middle of the count then, the count will stop and the output will be low. Therefore count resumes as the gate becomes high again. It in effect adds to the total time the output is low.

Mode 1: It is also termed as hardware triggerable one-shot. The triggering should be done through the GATE input through sending a 0-to-1 pulse to this. The following two steps should be performed as:

• Load that count registers.
• A 0-to-1 pulse should be sent to the GATE input to trigger that counter.

Contrast it with mode 0, wherein the counter produces the output immediately after the counter is loaded as long as GATE = 1.In this mode after sending the 0-to-1 pulse to GATE, OUT turns into low and stays low for a duration of N×T, after that becomes high and stays high till the gate is triggered again.

Mode 2: It is also termed as divide-by-N counter. In mode 2, if GATE = 1, OUT will be high for the N×T clock period and goes low for only one clock pulse, so the count is reloaded automatically, as well as the process continues indefinitely.

Mode 3: In mode 3 if GATE = 1, OUT is a square wave here the high pulse is equal to the low pulse when N is an even number. Then the high part and low part of the pulse have the similar duration and are equal to (N/2)×T (50% duty cycle). When N is an odd number, the high pulse is one clock pulse longer.

Mode 3 is extensively used as a frequency divider and audio-tone generator.

Mode 4: In mode 4 if GATE = 1, the output will go high upon loading the count. This will stay high for the duration of N×T. After this count reaches zero (terminal count), this becomes low for one clock pulse, after that goes high again and stays high till a new count or new command word is loaded. To repeat the strobe, the count should be reloaded again. Mode 4 is same to mode 2, except which the counter is not reloaded automatically. In mode 4, the count starts the moment the count is written in the counter.

Mode 5: It is similar to mode 4 except that the trigger should be done with the GATE input. In mode 5 after the count is loaded, we should send a low-to-high pulse to the gate to start the counter.