MPU speed question.

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metermannd

Joined Oct 25, 2020
178
I was looking at other Motorola 6800 family MPUs, curious to see if there was an 'off the shelf' equivalent of the SC44125 (customized 6800 with full-time A / D buses and the 6801 instruction set).

The 68HC11 has all the specific 6801 opcodes I would need (and which are not otherwise implemented on the 68HC05 or 6809).

I'm intrigued, but one thing: the HC11 clock speed is listed as 3MHz, while the SC44125 runs on a 1MHz clock.

Am I correct in assuming if I run the HC11 at its native 3MHz with the 6801 code, the code would end up running 3 times as fast?

It seems to be possible to tweak a PC to 'underclock' it to run at a lower speed, but has it been done with an 8-bit MPU?
 

BobaMosfet

Joined Jul 1, 2009
1,513
As long as you don't require time-dependency to be accurate, it will run at least 3 times faster. If you have code that is time dependent, it will need to be altered. Or, you can try running the MPU at 1Mhz external clock crystal and see what happens. It doesn't know how fast you're running it, the external clock is its reference. So long as it can do any housekeeping it needs to do in the time-frame and execute anything you want, it should work at 1MHz.
 

dl324

Joined Mar 30, 2015
12,681
Am I correct in assuming if I run the HC11 at its native 3MHz with the 6801 code, the code would end up running 3 times as fast?
Not necessarily. Architecture can have a significant impact on how long it takes an instruction to complete or whether a microprocessor can work on multiple instructions at the "same" time. Clock frequency comparisons are only valid between very similar architectures. You couldn't even do it with different versions of Pentium II because they always added features to make an existing architecture faster (e.g. more cache memory).
 

BobaMosfet

Joined Jul 1, 2009
1,513
Not necessarily. Architecture can have a significant impact on how long it takes an instruction to complete or whether a microprocessor can work on multiple instructions at the "same" time. Clock frequency comparisons are only valid between very similar architectures. You couldn't even do it with different versions of Pentium II because they always added features to make an existing architecture faster (e.g. more cache memory).
ooo_o0_ooo
 
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