3-Chip Z80 Design

Here's the old school front panel with modern twists. The front panel is controlled by a 32 bit RISC CPU over an I2C bus. Pushbuttons (cheaper than toggles) are used but made to function like toggles.

The external SRAM is tested from the PSoC. 

It works! Yeah...

GitHub repository for PSoC code.

Added test of all 512KB. Takes about 1 second.

I put in two chips backwards on the board. There's a big dot and a little dot. The little dot is pin 1. The big dot isn't.

After fixing that the PSoC programmed and I made the LED blink.

JLCPCB took less than a week. So quick I didn't order the parts before the PCBs arrived.

This is the schematic for the card as ordered.

Added terminators for the I2C (stuffing option). Added interrupt line for Front Panel design.

Added 2 pin header and RC filter for DAC output. 

Did board route glossing. Here's the design. Getting ready to order PCBs.

Oscillator and MCP23017 are optional. 

I ordered a batch of 10 boards from JLCPCB.

Final design has the following features:

• Z80 CPU - any speed from 4 MHz to 20 MHz should work
• 512KB Static RAM, 55 nS, Zero Wait State data and program memory
• PSoC5LP - Cypress Semiconductor System on a Chip
  • $8 part
  • 32-bit ARM CPU
  • 5V logic levels - all glue logic in PSoC
  • USB interface
  • I2C interface with interrupt input line
  • RTC with 3V 2032 battery
  • Serial connections with handshakes (could use external FTDI or RS-232 transceiver)
  • 8-bit DAC output
  • Emulates Z80 peripheral chips in PSoC firmware
  • Flexible Memory Mapper
    • Maps the 512KB SRAM into the Z80's 64KB space
    • Software-selectable bank sizes from 2K to 32KB
    • Can be configured "on-the-fly" or by compiling PSoC code for particular software
    • Z80 sets the banks with a write in I/O space - compatible with various software memory management schemes
• LED - controlled by PSoC
• Optional I2C Port Expander
  • MCP23017
  • Adds 16 bits of programmable I/O
• Optional oscillator
  • Not required for most applications since PSoC controls all interfaces and has an accurate trimmed internal oscillator
• Connectors
  • I2C
  • USB B
  • Serial TTL-level
• 95x95mm form factor with mounting holes