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My Stackable Z80 project...

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alank2:
I was originally thinking of making a SBC with SMD parts, but even at that I'm not sure if I could fit it all within 10cm x 10cm board like I'd like to.  In the meantime I definitely want to get it off of breadboards and onto some real pcb's so I decided to switch to making it a stackable project like the whole shield thing with arduino.  This would let me develop each one on its own, troubleshoot and/or correct each one on its own, etc.  I'm going to have a left side 2x20 header (all 40 z80 pins) and a right side 2x20 header (aux pins) and use stackable header to join all the boards together.  A board for the Z80, one for SRAM, one for the AVR, one for the serial ports, etc.

I plan on using an ATMEGA64A to eliminate many of the traditional Z80 SBC peripherals such as EEPROM (the AVR will preload SRAM), SIO, DMA, DISK (microSD via FATFS), RTC, etc.

Once I get at least the Z80, SRAM, and AVR boards done, I should be able to then just put smaller with considerably less items on breadboards using the headers easily.  That should reduce my bread boarding nightmare.

I've got the Z80 schematic done so far, but no board layout yet if anyone wants to review it (attached PDF).  It basically is a DC barrel power in with power switch that has some main caps and a TVS, the Z80 basically wired to the left bus (z80 bus), and the only pins it uses from the right bus (aux bus) are two signals to control the speed.  It will be software switchable between either 16/8/4/2 or 20/10/5/2.5 MHz controlled from the AVR so it has some logic to split and select the clock on board as well.  If you see any bugs or problems let me know!

Perhaps one day after everything is tested and mature, I'll try my luck at a single board version.

Bud:
Not sure if anyone would want to review an unreadable schematic.....it is a 101 example of how Not to draw schematics, sorry.

Ian.M:
There aren't enough grounds on the Z80 bus header so signal integrity will suck.

The original S-100 bus ran at a couple of MHz only had one ground pin, and as it was developed into an industry standard (IEEE Std 696-198) they had to steal back three rarely used signal lines to get good enough grounding to get it up to 10MHz.

If you want to keep the Z80 bus on a single connector, consider using 50 way headers to get enough spare pins for well distributed grounding.   I'd also use multiple paralleled pins for +5V, as otherwise you are likely to get problems with the +5V contacts overheating.

alank2:
Bud - I understand your comment - yes I took the easy way of connecting things with reference labels.

Ian - I couldn't get 50 pin connectors for any reasonable price (= they were not on eBay and cost 6 times as much at Digikey/Mouser).  I can move some signals to open spots on my aux connector to make room for grounds/vcc's every so many pins on the left one.  How many do you think would be smart?

alank2:
If I add more GND's, should the VCC's be balanced with them?  8 gnd's and 8 vcc's.  Or would 12 gnd's and 4 vcc's be better.

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