Even more compatibility
eZ80 for RC2014™ and compatible backplanes
An eZ80 CPU Module that works within the RC2014™ ecosystem
30 Jul 2024
In my previous post, I looked at how to improve compatibility with modules where the write logic is triggered on the positive edge of the WR signal. This is important for Modules such at the Compact Flash and banking Memory Modules. But I have since found I introduced an issue with the next module I attempted to test with; the SN76489 sound module by J.B. Langston.
This is a retro sound module based on the SN76489 sound chip. Its a fairly simple chip and given its age, its no surprise its a chip that can not be sent data, as quickly as the eZ80 is capable of achieving.
This chip only accepts writes - it does not allow its internal registers to be read. So the issue is only related to timing of data writes (eZ80 to SN76489).
It transpired, that one thing I forgot to consider, was the eZ80 WAIT input signal. This is a signal that the eZ80 (just like the Z80), can receive from ‘slow’ chips to cause the eZ80 to extend the writing (or reading) cycles. The SN76489 module connects the READY output to the CPU’s WAIT signal for just such a purpose. The logic I implemented in the ATF16V8 PLD did not take the WAIT signal into account. As such, it would deactivate the BUS-WR signal well before the SN76489 had time to load the data into its internal registers.
A quick bodge on the current PCB and an update to the PLD code, and suddenly I hear retro 8-bit sound coming from this old sound chip.
I did have to change the software a bit, to ensure the I/O is sent to the correct port still and also introduce a small delay between writes. All this code is in my branch of the RomWBW code.
Basically now, the logic is, if the WAIT signal is activated during a write cycle, the BUS-WR is not pre-released and is logically the same as the eZ80’s WR signal.
Once I got the SN76489 sound chip working, I found the next module I tested, the RC2014 YM/AY Sound Card worked without any more hardware modifications. Only required me to update the software again to ensure correct I/O addressing.
With the auto selection of the correct eZ80 bus cycles for I/O operations, I was able to play 8-bit chip-tunes with the TUNE.COM CP/M app, with the eZ80 overclocked all the way up to 32Mhz!