27C400/800/160/320 emulator/ROM switcher

[KenLowe]

Very impressive. If you're ever selling any of these, then I'll certainly be up for one.

[cmorley]

At the last ABUG I moved the ROM switch input on Tom's Master from the CPU (pin 40) to pin 13 of IC 19. This allows the module to hold the Master in reset under control from the USB serial link.

IC14 (CPU) pin 40 would still work but you'd be shorting the output of IC39 pin 1 to ground which isn't the kindest thing to do to TTL (won't break it but isn't kind!). Instead hooking the module in parallel with the BREAK key means it picks up BREAK presses but can also "press/hold BREAK" itself.

If you want to use the RESET via USB serial option I recommend the IC19 P13 connection.

The same could be achieved on the /PRST but you should fit a series resistor to the ROM select pin because you're shorting the 47uF cap to ground - 250R or greater would be OK to limit the ARM GPIO pin current to 20mA.

[cmorley]

OK... I found some enthusiasm and finished the "no reset wire" firmware.
v0.7 firmware dropbox link
v0.8 firmware dropbox link

You will need to update the microcontroller and the CPLD firmware.

After upgrading go in to the options and set them as you wish. Don't forget you can use the PRESET wizard to set the module up quickly. Also don't forget to COMMIT the options to FLASH.

ROM switch timings are a bit quicker now.
<1s - ignore
>1s - switch to NEXT ROM
>3s - swithc to ROM zero (optionally quick tap reset n times to go to ROM n)
>5s - no ROM switch

You can connect a (power) LED to the pad next to PIN 1. It is open drain so you connect the other end of the LED to 5v - use a series resistor so the microcontroller pin isn't current limiting.

The ROM switch without using a RESET wire works by monitoring A0. If it sees no change on A0 for a while (>30ms) it assumes the machine is in RESET. I have tested this on an Amiga A500. It might work on other machines (very probably most other 680x0 machines) depends on the CPU behaviour when in reset.

No RESET wire function hasn't been tested on a Master yet.

Edit: 0.8 has improved ROM switch input debouncing for more reliable wireless ROM switching.

[cmorley]

I've tested firmware v0.8 in my Master.

ROM switching without the RESET wire works

[Lilwashu]

Are there likely to be any more of these made or has the chip shortage done for them as they stand?

[cmorley]

No I am not going to make any more of these, sorry.

edit: the CPLD is impossible to buy and the module wasn't popular enough to bother reengineering.