Vlad
This is not a cycle-perfect zx spectrum compatible computer and, to be honest, it is not perfect at all. But it works and was able to run all games I've already tried. The intention was to create a computer using mostly technologies of years 1980-1990 so all components are THT except USB-C connector and jack connectors. And, of course, WeACT rp2350b Core board, but it is also kind of THT :)
No SD cards, no other speed-loading methods. You can only use your cassette recorder or smartphone to load/save programs via audio cable.
However, the video output is VGA and HDMI. VGA output uses 720x576p on 50Hz, horizontal lines are doubled to emulate ZX Spectrum video resolution. 576 lines is less than 625 PAL lines so I had to reduce bottom border to 48 lines instead of original 56.
HDMI is still in progress.
Note that almost everything Spectrum-specific is implemented programmatically in rp2350b firmware. In general you have classic Z80 computer with 16K ROM out of 64K 27c512 chip switchable by DIP-switches, 32K static RAM dedicated to Z80 and 16K static RAM shared between rp2350b and Z80. You can write your own ROM and your own rp2350b firmware to implement any kind of a video controller and input/output functionality. For example it may be a good challenge to use all 16K of shared RAM as a video memory and emulate CGA video modes like 320x200 etc. Also there is an expansion connector having all Z80 signals. I used it to connect kempston-like interface for old-style GamePort joystick but it can be used for any external device.
Software sources and KiCAD projects can be found in my git repository https://github.com/kysilv/z80-rp2350b-zx-compatible
The project is in progress. Initially I made several mistakes in schematic and even in PCB so first version of working device doesn't look very nice. KiCAD projects have been fixed but fixed version is not implemented "in metal" yet.
Hardware
(software/firmware description added below)
Current schematic:
The ROM/RAM part is quite simple:
Here is 32K static RAM chip HM62256 and 64K EEPROM 27c512 sliced to 16K banks that can be switched using DIP-switches. /ROMCS signal exposed to an expansion connector and can be overwritten by external device.
Video RAM and rp2350b playing "ULA" role are here:
I've tried to separate "ULA" data bus and CPU data bus by resistors only, same as in original ZX Spectrum and it works perfectly. To separate address bus and RD/WR control signals I used buffers 74ls541 controlled by rp2350 software. After some timings adjustment it works too...
CPU clock signal is generated by rp2350b according to the memory contention scheme and amplified by transistor s9018 identically to the original ZX Spectrum schematic.
CPU RESET is initially controlled by rp2350b GPIO15 during startup but also there is a button for manual reset afterwards.
Connectors j6 KB1 and j7 KB2 are for keyboard. J5 Joystick is for Sinclair joystick 1 which is just parallel to keys 6,7,8,9,0. I intentionally selected some 9-pin d-sub connector footprint to get the pads on PCB and decide later on actual pin mapping for some physical joystick that I don't have yet.
There are two 5V power connectors just for convenience so you can use USB-C or barrel jack whatever power supply you have.
Video outputs VGA and HDMI:
Resistor/diodes networks schematic I found here: https://github.com/fruit-bat/pico-zxspectrum
My initial idea was to automatically detect what connector the monitor is connected to and load corresponding PIO ASM code to support VGA or HDMI. Hopefully it will be possible to detect monitor using ADC of rp2350b. Currently only VGA is implemented.
HDMI is still under investigation.
Couple photos:
I can't recall if black color should be changed by BRIGHT command to grey... If yes, then this is an issue to think and fix..
Cassette/audio interface is probably the worst part of the schematic:
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István Hegedűs
Steve Toner
Clyde Shaffer
256byteram