z80+PSoC5 – Z80 SBC on a Stick

A new iteration of the concept : Romless z80sbc with a PSoC5 module that is doing bootloader/glue logic/serial output & sd-card interface.

This time I designed a PBC for the project (OSH Park)

The BOM is :

• Z80 CPU (plcc)
• 128k Sram
• some resistors (resistor arrays)
• some capacitors (decoupling)
• a microSD card breakout board (SV2 on schematic)
• a PSoC5 module (cy8ckit-059, around 10 euros)

Some explanations :

• Almost all signals are interconnected between Z80 and PSoC5+SRAM
• but since I/Os are limited on PSoC5 module, only A0-A5 and A14-15 are in fact connected.
  • It limits IO port decoding since A7 & A6 are not present, thus OUT to port 0xA8 would be seen as OUT to 0x28… not a big problem with the Roms I wanted to test.
  • A6-A13 are pull-down, this is important for Bootloader (see just after)
  • When a Z80 access is done on an IO port, WAIT signal is used to give time to the PSoC to handle the request (Bootloader byte / Serial In/Out / SDcard access / Interrupt ?)
  • Memory accesses are done directly to the SRAM, without PSoC intervention – only a LUT table is used for the memory bank switching (normal mode is 0x0000-0xFFFF mapped to RAM, bank 1 maps RAM 0x10000 to 0x4000-0xBFFF, bank 2 maps 0x18000 to 0x4000-0xBFFF)
• The design is ROMless, so a specific bootloader process is used to fill the RAM at boot :
  • The Z80 is put in RESET mode – during RESET, Address bus and Data bus is in high impedance mode
  • RD and WR are still driven, so a resistor is needed between the Z80 and Ram/PSoC to allow the PSoC to drive those signals during the RESET/Bootloader phase.
  • PSoC inputs (A0-A5, Data, RD/WR) are dynamically changed to Outputs – A6-A13 will be pulled low – B0-B2 will be forced low too (bypassing the LUT output), so A14-A16 on the SRAM will be low. Read/Write to SRAM will be limited to 0x0000-0x003F (64 bytes).
  • A small Z80 program is directly written to RAM
  • There is a verification phase (Data are switched back to inputs on PSoC5) to check the bootloader.
  • The Z80 is put out of RESET, and starts running from 0x0000…
  • The bootloader program relocates itself from 0x0000-0x00xx (xx=~24) to 0x4000, and gets additional bytes from port 0xFF (it writes them from 0x0000 to 0x3FFF – if less than 16K, then ‘xx’ is output).
  • At the end, bootloader jumps back to 0x00
  • the Z80 now executes the code it received on port 0xFF…
• IO access to serial chips (SIO/ACIA) is translated by the PSoC to UART commands
• IO access to storage (for CPM 2.2/3.x) is translated by PSOC to FatFS access on cpm.bin or cpm3.bin read/writes – thus is is quite easy to change/create disks as it is only a file on the uSD on a regular FAT filesystem.
• Interrupt using IM2 and SIO is supported (needed for Grant Searle mode)
• At boot, the PSoC program waits for some basic commands on the console: at the moment, ‘1’ , ‘2’ , ‘3’ are selecting Grant Searle Rom, RC2014 Rom or CPM3 test ‘cpmldr.com’ respectively (each one with its specific code for IO handling/main loop), ‘B’ installs the small bootloader code on the SRAM and ‘R’ starts the Z80 by releasing RESET.

On this platform I was able to run, without re-programming to PSoC and using the same sdcard :

• Grant Searle’s ROM with Basic & CPM2.2 (without modifications)
• RC2014 Basic Rom
• A preliminary test CPM3 based on sources from S100computers.com

I was able to have a stable system @ 16Mhz, with a regular Z84C0008VEG (8Mhz) and all ZEXALL tests passed in ~49 minutes.

    

   

   

Cypress PSoC Creator project file :

z80smd.cyprj.Archive01

Eagle Schematic and Board :

z80smd

Notes on HW/PCB :

• BUG : a pull-up resistor is missing on Z80 BUSREQ signal.
• C7 on the PSoC module needs to be removed (that capacitor was put for analog
• On uSD breakout board, I had to change R4 to make it a pull-up on MISO line (found a schematic on http://file.vishnumaiea.in/hard/micro-sd/Catalex-mSD-Card-Module_SCH.png)

Misc. Data to put on uSD :

Not yet released… need to check if redistribution is allowed