REV4 Complete

A project log for z80ctrl

AVR-based bootloader and IO card for RC2014 retrocomputer

J.B. LangstonJ.B. Langston 12/05/2018 at 03:170 Comments

I have completed REV4 of the z80ctrl board. This revision makes the following changes:

Adds a diode on the /WAIT line to convert it to an open drain output. This is necessary to prevent contention when other boards (such as my SN76489 sound card) need to use the /WAIT line also.  If you plan to use a REV3 board with the SN76489 board, you must cut the wait trace and solder a diode in its place to prevent contention.

Updates the wait state generator to allow debugging with an external clock. Previously, z80ctrl only generated wait states for IORQ and when debugging, it single stepped the Z80's clock in software.  Now it can optionally also generate wait states for MREQ so that execution of the Z80 can be paused for each memory fetch to allow debugging when the clock cannot be manually controlled.  A jumper (J9) is provided to select between CLK and WAIT modes.  In CLK mode, pin 20 of the AVR is connected to the CLK line on the RC2014 bus, and the z80ctrl provides the Z80's clock like it has in all previous revisions.  In WAIT mode, an external clock must be used and pin 20 is instead used to enable and disable wait states for MREQs. I have not yet written the software to support this mode of operation but the REV4 board will work the same way as REV3 and earlier when the CLK jumper is shorted. Important Note: U3 is now a 74HCT02 quad NOR gate instead of a 74HCT74 dual flip-flop used on previous revisions.

Adds a jumper to optionally connect the AVR reset pin to the D15 line on the RC2014 bus, which is normally unused. This line can also optionally be connected to the I/O expander's reset pins on the new I/O expander board, allowing the I/O expanders to be reset to a known state whenever the z80ctrl is reset.