Simple Question: 65c816 Memory Decode Logic

[BigDumbDinosaur]

Dr Jefyll wrote:

Alright, here's a bare-bones schematic, based on the discussion above...Edit to the diagram: remove inverter between A15 and /CE,

That could be further reduced to a single quad NAND. Jus' sayin'...

[Jmstein7]

BigDumbDinosaur wrote:

Jmstein7 wrote:

But, what wires connect to /OE and /CE (or /CS) on the chip? I.e., what wires get connected to Output Enable, Chip Enable, and, I guess, /RW?

Sorry for the earlier terse post. My wife called me to dinner right as I was posting and around here, a juicy chuck roast with corn-on-the-cob has priority over computers.

No problem - I fell asleep while putting my kids to sleep. Just got up, heading to work. To be continued!

Jon

[Dr Jefyll]

As noted earlier, there's more than one "right answer" to the question. Here again is the "classic" schematic -- the arrangement which uses a pair of NAND gates to generate qualified /Read and /Write signals from Phi2 and the CPU's R/W output. I added the Truth Table for typical RAMs.

Now here's a schematic which, by comparison, has both advantages and disadvantages. Therefore it's reasonable to make your choice according to prevailing priorities and circumstances.

The classic circuit requires more gates, but it may be a better choice if you're keen to run an old-ish RAM at the highest possible speed. /CE is what "wakes up" the chip and moves it from "power-down" to "selected," and the classic circuit allows /CE to go true ASAP. Then later when Phi2 rises we get the actual Read or Write. These four modes are shown in the Truth Table (and the "classic" version uses all four modes)

The new circuit is simpler but the RAM remains in "power-down" until Phi2 rises; then it transitions directly into either Read or Write. (The "selected" mode never occurs.) The lack of an advance wake-up results in a performance penalty which you may or may not deem significant.

It may seem odd that /OE is grounded; however, in the Truth Table you'll see that the state of /OE doesn't matter (it's a "don't care") if /WE is low. /WE and /CE alone are sufficient to select the three modes we need: power-down, read and write.

( Whether you do or don't choose the classic circuit is a general decision -- it has no specific bearing on the use of the '816 as opposed to '02 or 'C02. )

-- Jeff

[Jmstein7]

I laid this out late last night. I just have to label what everything is



edit: I already see an error. Abort isn't tied high.