Author Topic: Double-checking address decoding  (Read 564 times)

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Offline metertech58761

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Double-checking address decoding
« on: October 27, 2021, 02:09:57 am »
So I ended up choosing a replacement MPU for my tester project, and along the way, I had to redo the address decoding for the I/O ranges.

Can anyone take a look and see if I did it correctly?
 

Online MegaVolt

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Re: Double-checking address decoding
« Reply #1 on: October 28, 2021, 08:28:10 am »
Code: [Select]
A15 A14 A13 A3 A2
 1                 U4
 0   0   1   1     U2
 0   0   1   0     U3

If I understand correctly, the choice of chips is correct.
The polarity of signals A00 and BCS is not very clear? Active 0 or 1?
 

Offline metertech58761

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Re: Double-checking address decoding
« Reply #2 on: October 28, 2021, 11:37:05 am »
The 8CS line is the active-low chip select line for the 8279 IC on the display board, and disregarding shadow copies, should correspond to 0x2008-0x200B.

A00 is the mode select line for the 8279; when address 0x2008 is selected, this line should be low, putting the 8279 in data mode. When 0x2009 is selected, this line should be high, putting the 8279 in command mode.
 

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Re: Double-checking address decoding
« Reply #3 on: October 28, 2021, 12:08:22 pm »
Here is the project in the simplest simulator. You can call the truth table and check if it works correctly.

I see the simultaneous selection of BCS and A00 simultaneously with the selection of chips U2...U4. I don't really understand if this should be the case.

123.dig.txt rename to 123.dig

Simulator https://github.com/hneemann/Digital
 

Offline metertech58761

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Re: Double-checking address decoding
« Reply #4 on: October 29, 2021, 10:31:11 am »
I guess I was a bit too optimistic to squeeze the 8279 in like that. :)

I may have to rearrange the coding to move the 8279 down to 0x2800 / 0x2801 instead.
 

Offline metertech58761

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Re: Double-checking address decoding
« Reply #5 on: November 01, 2021, 11:40:26 pm »
OK, so I ended having to kick the 8279's 8CS and A00 lines further down to 0x4000 / 0x4001 (and swap the two addresses in firmware).
 

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Re: Double-checking address decoding
« Reply #6 on: November 02, 2021, 07:44:23 am »
Glad to hear that the issue is resolved :)))

What program was used to draw the schematic?
 

Offline metertech58761

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Re: Double-checking address decoding
« Reply #7 on: November 02, 2021, 12:29:55 pm »
KiCad. :)

Now that it seems the design of this board has finally settled into place and I have the HC11 chip on hand, I can begin layout of this PCB.

Next up - the redesign of the display / keyboard PCB (want to replace the 8279 with something more modern).
 


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