CTC

The CTC has been temporarily reduced to four channels from eight.

Eight independent CTC channels are available on ports 0x183B through 0x1F3B. These perform counter / timer functions that can be used to generate timer interrupts or to generate interrupts on behalf of physical signals.

The CTC is a standard Zilog part. Its datasheet can be found at http://www.zilog.com/docs/z80/ps0181.pdf . The Zilog documentation is ambiguous around how soft resets are treated so the following clarifies some points in the Next's implementation.

1. Hard reset requires a control word to be written with D2 = 1 (time constant follows) otherwise the channel effectively ignores the control word and will remain in the hard reset state.

2. Soft reset is generated when the control word's D1 = 1. if D2 = 0, the channel will enter the hard reset state. If D2 = 1 the channel expects a time constant to be written next and after that the counter/timer will run as expected.

3. Changing the trigger edge selection in bit 4 counts as a clock edge. A timer waiting for a clock edge to start will start and in counter mode, the count will be decremented.

4. ZC/TO is asserted for one clock cycle and not for the entire duration that the count is at zero.

To soft reset a particular channel that is in an unknown state, a soft reset control word should be written twice with D2 = 0 (no time constant follows).

At the moment, the ZC/TO output of each channel is fed into the CLK/TRG input of the succeeding channel so that time and count periods can be cascaded. Channel 3's ZC/TO output is divided by two and drives the joystick's clock in io mode.