There are four hardware timers (plus the clock chip)-one is 8-bit and three 16-bit:
q The 8-bit timer can be clocked internally at 814ns, 6.51�s or 104ms or it can use an external clock enabling it to be used as an external event counter.
q The 16-bit timers can be set to internally increment every 407ns, 814ns or 3.255�s and any of the three can use an external clock.
Counter-timer pins are mainly on Port 7, which has Schmitt trigger inputs. This means that the signal you provide need not have sharp edges and full logic levels. See PARALLEL INPUT & OUTPUT, page 74, for voltage specifications. The CIRCUIT DIAGRAM, page 40, and PIN CONNECTIONS, page 38, will help you allocate timer-counter pins in your application, but see the Hardware Manual sections 10 (16-bit timers) and 11 (8-bit timers) for full information on how to use them.
All four timer-counters can be read and written by software and each may generate an interrupt on overflow to allow extension to 32-bit counting and beyond. Every timer has two output capture registers that can raise a flag when the counter matches the output capture value. The 8 flags can be polled, used to cause interrupts or generate a hardware output. Each of the three 16-bit timers also has an input capture mode-on receipt of an external signal the counter value is transferred to a 16-bit register so that the exact transition time can be read by software.
This table shows how supplied software uses the timer-counters: