Friday, April 21, 2017

The Z80_mini Rev B

I had previously written about how to set up the standard Z80_mini for use with an FTDI-style USB-Serial cable.

This article will describe what else I have done with the Z80_mini.

To recap, the Z80_mini is a little Z80-based single board computer designed by Bernd Ulmann and described in some detail on his website. I like the simplicity, flexibility and completeness of the design, including the use of a 16550 UART, arguably one of the best old-style UARTs out there, a generous use of decoupling capacitors and separate crystal oscillators for the UART baudrate and CPU clock.

Best of all, Bernd has made the Eagle design files available on this website. Which leads to very interesting possibilities.

First of all, I was able to send the .brd file to OSH Park to get myself a set of (3) professionally produced Z80_mini PCBs.

Second, while I was able to, with some minor modifications to the board, adapt it for use with an FTDI USB-serial cable, I could go even further than that.

Eventually, I decided to redesign parts of the Z80_mini PCB. Hence the Z80_mini Rev B!!!

The original Z80_mini is on the right. The redesign Rev B design is on the right.

A picture of an assembled board is below.

The obvious changes are the elimination of (a) the power connector, (b) the circuitry around the MAX232 and serial connector and (c) the addition of the 6-pin FTDI connector.

The less obvious changes are the addition of jumpers to allow the use of 28C256 EEPROMs, a little pin header to bring out the OUT1 and OUT2 signals of the UART e.g. for an audio output perhaps...

The hidden changes involve adding the M1 signal to the UART chipselect, wiring the CTS line to ground (in case a 16C550 is used and we want to run the UART with autoflow control) and of course, routing the RTS signal to the FTDI connector.

Using 74ACT devices, 55ns RAM, 150ns EEPROM and a 20MHz Z80, I've been able to run the board at the full 20MHz spec of the CPU. Instead of Bernd's original 1.8432MHz UART clock, I chose to use a 7.3728MHz clock instead and ran the serial port at 115200 a la RC2014.

This is a very speedy Z80 indeed!

For the adventurous, the Eagle board design files are in GitHub. Z80 assembly language source files are available for a BIOS+Monitor/Debugger+BASIC combo at GitHub. A binary can be found in the rom/ folder if you don't want to muck around with a cross-assembler.

Finally, you can directly order boards at OSH Park with the link below.

Do note that I do not get any money from OSH Park should you decide to order a set of boards through them. The project sharing at OSH Park is primarily so that you don't have to load the .brd files yourself.

Order from OSH Park

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