processor "speed" ratings, specifications

[Twospruces]

I have been spending some time modifying the TRS-80 model 100 to run at 2x clock IE 4.9152MHz.
The production 80C85 specification clearly does not support such operation.
And yet, it works well, under some conditions
* you need to use fast RAM
* you need to use fast ROM
.. and here fast means <200nsec. I use 120nsec SRAM and 150 nsec EPROM.


It occurs to me that, if one were writing specs for a processor, that they must have assumed something about the responsiveness of memory.
If Intel assumed "typical or slow" memory, then that would clearly reduce max speed, whereas in the timing budget they could assume that memory was fast, perhaps that supports a higher clock rate.

Is there some rule of thumb around this? What do processor vendors assume about memory speed when they are rating their devices?

I've been experimenting with different speed ratings on processors too. It is clear that processors with faster ratings can sustain higher speeds (As expected) but they all seem to run a fair bit faster than their ratings suggest.

thx

 

[gslick]

It occurs to me that, if one were writing specs for a processor, that they must have assumed something about the responsiveness of memory.
If Intel assumed "typical or slow" memory, then that would clearly reduce max speed, whereas in the timing budget they could assume that memory was fast, perhaps that supports a higher clock rate.

Is there some rule of thumb around this? What do processor vendors assume about memory speed when they are rating their devices?

When writing specs for a processor, the processor vendor just needs to publish detailed timing specifications as a function of the processor clock cycle period. Then it is the responsibility of the system designer to choose other system components with timing specification that are fast enough to meet the processor timing specifications at the desired processor clock cycle period.

For example this OKI databook has timing specifications for the 5MHz rated MSM80C85A-2
http://www.bitsavers.org/components/oki/_dataBooks/1990_OKI_Microprocessor_Data_Book.pdf

Page 61 (page 66 of the PDF) has a table of A.C. Characteristics. Page 62 has a Bus Timing Specification as a Tcyc Dependent table. Page 63 has timing diagrams for Read and Write Operations with visual representations of the various timing parameters.

 

[Twospruces]

thanks. yes I have looked at it (alot) and I guess my impression is the specs they publish don't answer the "what if" scenario. meaning, for example, if RAM was significantly faster than the minimum requirement, can that support an increase in clock rate.

The specs say - for the fastest clock I specify, memory has to be at least this fast. as dictated by the various worst case timings that the processor must see.
The other way to look at it is - with the fastest ram possible what is the minimum clock cycle supportable?

And yes I know there is a 5MHz rates 80C85. Interestingly it was measured to be no faster than the stock 3MHz rated part. IE they both tolerated about the same overclocked rate.

There is also an 8MHz rated Tundra 80C85. That one has materially faster performance.

In my testing, I had a crude clock synthesizer set up to drive clock rates in steps, so I could ratchet up the clock rate and measure when the device "hit the wall". It was pretty easy to tell when the processor started to see errored memory read/writes.