I've recently designed an I/O expander companion board for the z80ctrl that adds an RTC, up to 4 8-bit GPIO ports, and 4 SPI ports.
The board makes use of the same MCP23S17 I/O Expander used on the z80ctrl. All of the I/O expanders share the same chip select line and are addressed by a configurable 3-bit address, allowing a total of 8 I/O expanders to share a chip select line. Jumpers are provided to set the address, so up to 3 I/O expander boards can be added to the RC2014 if desired, giving a maximum of 24 8-bit GPIO ports. The I/O expander on the z80ctrl board which interfaces with the RC2014 bus is hard-coded to address 0, so only address 1 and higher should be configured for the I/O expanders on this board.
The board also contains a DS1306+ SPI RTC with a battery backup and 32 kHz crystal. This allows the AVR to keep track of the time in order to correctly timestamp files on the SD Card. It can also share the RTC over the Z80's parallel bus so that the RTC is accessible to CP/M, FUZIX, or other operating systems running on the Z80.
The board allows for up to 4 additional SPI chip selects to connect other SPI peripherals to the board. This is done by ORing the upper 4 bits of the GPIO_B port with the AUX2_CS line from the AVR. The additional chip select will only become active when both corresponding the GPIO_B line and the AUX2_CS line are set low. This way, the AVR can communciate with the I/O expander to enable the additional chip selects, but they will only become active once communication with the I/O expander is complete.
An inverter was also required because the RTC uses an active high chip select whereas most SPI peripherals use active low. Since I had 5 additional inverters in the package, I also inverted the outputs of the OR gate and exported both the active high and active low chip selects on each of the 4 SPI ports.
I have updated the z80ctrl firmware to use the RTC to set the timestamp when creating new files on the SD card. The date can also be viewed and set using the new date monitor command.
I have also added ioxwrite and ioxread monitor commands that directly write or read I/O expander registers. This allows low-level configuration of each I/O expander. The commands take the address of the I/O expander configured via the jumper, and the hex address of the register that you want to interact with. Refer to the MCP23S17 datasheet for information about the specific registers. Additional high-level commands can be added to the firmware if a specific function for the I/O expanders is desired.
Many thanks to JLCPCB for sponsoring the prototypes of this board. Whether you want to manufacture my boards for your RC2014, or you need to prototype your own electronic project, JLCPCB is an excellent choice. They produce top quality PCBs for an incredibly low price and their service is fast. I routinely get 5 day or quicker turnaround on my projects, order placed to board in hand.