Smonson's slow H4 build

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exxos
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Re: Smonson's slow H4 build

Post by exxos »

stephen_usher wrote: Fri Mar 26, 2021 12:02 pm Does this give sufficient delay though?
You don't need to delay it, not unless you're using a booster to speed up ROM ( but that can be fixed in the booster anyway) , as the CPU will not pick up the data for 100+ns anyway.
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Re: Smonson's slow H4 build

Post by Smonson »

exxos wrote: Fri Mar 26, 2021 11:53 am You don't need to bother with LDS/UDS for ROM.
stephen_usher wrote: Fri Mar 26, 2021 11:59 am Indeed, UDS and LDS are for writes only.
What about /OE though? Just tie it to ground?
exxos wrote: Fri Mar 26, 2021 12:00 pm If you are just using U4 for DTACK.. I really would not bother, just use a diode and tie to ROM_OE...
Yes, I believe so. That part is from the schematic provided by the original author (Ted Rossin). By ROM_OE, do you mean LDS/UDS?
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Re: Smonson's slow H4 build

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Smonson wrote: Fri Mar 26, 2021 12:21 pm What about /OE though? Just tie it to ground?
You can tie CE to GND.. OE is used to enable the ROM.. then put a diode to The same signal with a diode then you can pull DTACK low as well.
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Re: Smonson's slow H4 build

Post by stephen_usher »

Smonson wrote: Fri Mar 26, 2021 12:21 pm
exxos wrote: Fri Mar 26, 2021 11:53 am You don't need to bother with LDS/UDS for ROM.
stephen_usher wrote: Fri Mar 26, 2021 11:59 am Indeed, UDS and LDS are for writes only.
What about /OE though? Just tie it to ground?
I used the GAL to do the address decoding, which controls /OE and the /AS line to run /CE on the ROM. (This matches the timing diagram as /CE should be before /OE.) This meant that the ROM was primed to output whenever an address appeared on the bus. It was then up to the GAL to decode the address or /ROM2 along with /RW to determine if it's a read operation so as to enable the output from the ROM (/ROE in my schematic). This same signal was used to enable /DTACK via the 74LS244.
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Re: Smonson's slow H4 build

Post by Smonson »

exxos wrote: Fri Mar 26, 2021 12:35 pm You can tie CE to GND.. OE is used to enable the ROM.. then put a diode to The same signal with a diode then you can pull DTACK low as well.
Ah, got it. Like this, yeah? (ignoring the signal naming of "CE" which is now misleading)

256krom-2.png
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Re: Smonson's slow H4 build

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stephen_usher wrote: Fri Mar 26, 2021 12:40 pm
I used the GAL to do the address decoding, which controls /OE and the /AS line to run /CE on the ROM. (This matches the timing diagram as /CE should be before /OE.) This meant that the ROM was primed to output whenever an address appeared on the bus. It was then up to the GAL to decode the address or /ROM2 along with /RW to determine if it's a read operation so as to enable the output from the ROM (/ROE in my schematic). This same signal was used to enable /DTACK via the 74LS244.
I could go down the custom route using a cheap CPLD, but using "F" high-speed logic seemed a lot more bullet-proof. I also can't stand soldering the EQFP packages on MAX Vs :)
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Re: Smonson's slow H4 build

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Smonson wrote: Fri Mar 26, 2021 12:51 pm Ah, got it. Like this, yeah? (ignoring the signal naming of "CE" which is now misleading)
Yep!

But CE should be OE on the diode signal :)
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Re: Smonson's slow H4 build

Post by Smonson »

exxos wrote: Fri Mar 26, 2021 12:54 pm Yep!

But CE should be OE on the diode signal :)
Sweet, thanks! Let's just pretend that says OE there. It's just a net name!
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Re: Smonson's slow H4 build

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Smonson wrote: Fri Mar 26, 2021 12:53 pm but using "F" high-speed logic seemed a lot more bullet-proof.
For this particular application, I really would suggest using slower LS types... With them being lower power and slower they are a lot less "noisy".
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: Smonson's slow H4 build

Post by Smonson »

exxos wrote: Fri Mar 26, 2021 12:58 pm For this particular application, I really would suggest using slower LS types... With them being lower power and slower they are a lot less "noisy".
Alright, I'll take your advice on that too. Thanks again!
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