The WonderSwan features two timers:
- Horizontal Blank Timer - counts down every horizontal blank (256 CPU cycles),
- Vertical Blank Timer - counts down every vertical blank (beginning of line 144).
The timers also feature an auto-reload functionality: that is, they can be configured to fire one time or repeat periodically.
Each timer has its own interrupt; it is triggered when the counter would be about to count down to zero, that is when the counter's value is 1 and the timer condition occurs.
This leads to a quirk: the timer interrupt will be triggered if the reload value is set to one even if the timer countdown is disabled, so long as the interrupt is enabled.
Timer Control ($A2, $A3)
7 bit 0 ---- ---- .... VvHh |||| |||+- Horizontal blank timer: Enable countdown. ||+-- Horizontal blank timer: 0: One-shot, 1: Repeat/auto-reload. |+--- Vertical blank timer: Enable countdown. +---- Vertical blank timer: 0: One-shot, 1: Repeat/auto-reload.
Horizontal Blank Timer Reload ($A4, $A5)
Vertical Blank Timer Reload ($A6, $A7)
Contains the initial counter value. The Timer Counter port is immediately initialized with this value upon writing. If auto-reload is enabled, the interrupt trigger will re-initialize the Timer Counter port with this value.
Horizontal Blank Timer Counter ($A8, $A9 read)
Vertical Blank Timer Counter ($AA, $AB read)
Contains the current value of the counter.
Note that if auto-reload is enabled, this will never contain the value zero.