Mode of operation
The value of this counter is decremented by the pixel clock, which runs at 6MHz (166.7ns period). It is automatically reloaded with the value specified in REG_TIMERHIGH and REG_TIMERLOW each time one of these 3 selectable events occur:
- Immediately when REG_TIMERLOW register is written to.
- At the beginning of the frame blanking period (one-shot).
- When the counter reaches 0 (repeat).
- Minimum time ($00000000): 166.7ns, corresponds to 1 pixel, useless, see below.
- Maximum time ($FFFFFFFF): 715.8s, corresponds to 42366 frames, or 11.9 minutes.
- Cartridge systems: $68 (IRQ2)
- CD systems: $64 (IRQ1)
Like other interrupts, this one needs to be acknowledged with REG_IRQACK to trigger again.
The REG_LSPCMODE register is used to configure the timer's operation:
- Bit 4 = 1: Enable timer interrupt.
- Bit 5 = 1: Reload counter as soon as REG_TIMERLOW is written to.
- Bit 6 = 1: Reload counter at the beginning of the hblank of the first vblank line (start of each frame).
- Bit 7 = 1: Reload counter when it reaches 0.
Caution: When the timer interrupt is enabled, the value of REG_TIMERHIGH and REG_TIMERLOW should always be set higher than 4, or the 68k will get flooded with interrupts and program execution will lock up.
Example: Neo Turf Masters
- A: First line of the frame. The timer is reloaded with the value set at C: 41016. The next interrupt will trigger at B, in 106 lines and 313 pixels (just before the 107th line). The scroll value for the mountains is set.
- B: Start of the ground area. The timer is immediately reloaded to 767 and the mode is set to "reload when 0 is reached". This will trigger interrupts every 2 lines so that the ground can be shifted to produce the perspective effect. A variable is used to count the number of ground lines.
- C: End of the ground area. When enough ground lines are rendered, the timer is immediately reloaded to 16383 so that the next interrupt won't happen soon. The mode is set to "reload on first line of next frame" and the reload value (not the timer !) is set to 41016.