Editing MOS Technology 6522 (section)

==Timers==
The VIA provides two 16-bit timer/counters.  Each can be used in one-shot ([[monostable]]) "interval timer" mode; timer 1 can also be used in "free-running" (divider/[[Square wave (waveform)|square wave]]) mode, in which the timer is automatically reloaded with the initial count when it reaches zero, and timer 2 can also be used in "pulse counting" mode, in which the timer will count the high-to-low state transitions of pin PB6 (the 7th bit of parallel I/O port B).  In the one-shot mode, each timer continues free-running so that the time since zero was reached can be determined, but no further interrupt is issued until the timer is restarted.  Each timer can generate an interrupt when it reaches zero, and timer 1 can also output pulses (in the interval timer mode) or square waves (in the free-running mode) on pin PB7 (the 8th bit of port B).  Timer 2 can be used to provide the clock frequency for the shift register.  A useful feature of the free-running mode is that the 16-bit counter latch can be loaded with a new count without reloading the counter, so that it will load the new count only after the current count reaches zero, seamlessly changing the output frequency.  This feature of timer 1, combined with its ability to output to pin PB7, can be used to generate complex waveforms, for example [[pulse-width modulation]] signals, frequency sweeps, or [[bi-phase encoding|bi-phase]] or [[Differential Manchester encoding|FM]]-encoded serial bit streams.<ref>1982 MOS Technology Data Catalog, pgs. 2-54 and 2-55</ref>