Programmable Interval Timer:

There are 3 counters (or timers), which are labelled such as Counter 0, Counter 1 and Counter 2. Each of counter contain 2 input pins - CLK (clock input) and GATE - and 1 pin, OUT, for data output. The 3 counters are 16-bit down counters independent of each other, and can be read easily by the CPU.

The first counter (selected by setting A1 = A0 = 0, see Control Word Register below) helps produced an 18.2 Hz clock signal. The second counter (A1 = 0, A0 = 1) assists in producing timing, which will be utilized to refresh the DRAM memory. The last counter (A1 = 1, A0 = 0) generates tones for the PC speaker.

Besides the counters, a typical Intel 8253 chipset also contains the given components: