Intel 8253/54 - Operational Modes

8253/54 can be operated in 6 different modes. In this chapter, we will discuss these operational modes.

Mode 0 ─ Interrupt on Terminal Count

It is used to generate an interrupt to the microprocessor after a certain interval.
Initially the output is low after the mode is set. The output remains LOW after the count value is loaded into the counter.
The process of decrementing the counter continues till the terminal count is reached, i.e., the count become zero and the output goes HIGH and will remain high until it reloads a new count.
The GATE signal is high for normal counting. When GATE goes low, counting is terminated and the current count is latched till the GATE goes high again.

The output is normally high after initialization.
Whenever the count becomes zero, another low pulse is generated at the output and the counter will be reloaded.

This mode is similar to Mode 2 except the output remains low for half of the timer period and high for the other half of the period.

In this mode, the output will remain high until the timer has counted to zero, at which point the output will pulse low and then go high again.
The count is latched when the GATE signal goes LOW.
On the terminal count, the output goes low for one clock cycle then goes HIGH. This low pulse can be used as a strobe.

This mode generates a strobe in response to an externally generated signal.
This mode is similar to mode 4 except that the counting is initiated by a signal at the gate input, which means it is hardware triggered instead of software triggered.
After it is initialized, the output goes high.
When the terminal count is reached, the output goes low for one clock cycle.