Zalt: The Decoder Board (again)

After I had finished my hand-wired Bus-Spy board, I turned my attention to the Decoder board. I had already assembled the Decoder board PCB with the clamp diodes and resistors I needed to bring the 5V signals down to 3V3 and visa versa. There is documentation on the Altera site on how to do this.


This is the Decoder board as it came from Elecrow. This time I opted for the green solder mask just to see what the quality would be. Again, Elecrow has delivered excellent work, although this time, on few via’s, the hole was a little off center. No break outs or anything, so not a problem.

I had already ordered all the passive SMD diodes, resistors and capacitors that needed to go on the board. After a few hours soldering with my magnifier (old eyes) it looked like this.


As you can see, I did not populate everything just yet, although I did solder all the diodes. This is because of the 80 IO pins the Altera Max II (EPM240) has, I only needed about two-thirds. The rest is will be done as needed. For the unconnected IO pins I designed both a pull-up resistor for output as well as a series resistor for input. Because I did not solder the resistors for the unconnected pins, those pins are physically isolated from the header pins they connect to.

After I had the Bus-Spy board and the CPU board running, I could do DMA from the System Controller (on the Bus-Spy board) and run a simple ‘Hello World’ program from RAM I added the Decoder board to the stack. Without the Decoder board, I had to use a jump wire to connect the enable for the RAM chip to the Z80 MEMREQ control line in order for it to work – otherwise the (one and only) memory chip would never be enabled.

In order to start small and test if the Max II could even be programmed I coded the following VHDL:


Sheer brilliance if you ask me. Basically the jump wire in code: let the first (zero) Memory Bank Enable be the same as the Z80 MEMREQ.

I was pleasantly surprised when the Quartus software programmed my board on the first try and my simple “Hello World” test program ran once again. That meant that the Decoder board was basically working! Whoohoo!

The tower of power was growing:


Published in: on June 26, 2016 at 7:14 am  Leave a Comment  

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