Date   : Wed, 14 Apr 2004 12:16:25
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: OVERCLOCKING

"JAMES WATSON" <james@...> wrote:

NO NEED TO SHOUT!

In fact, one of the most common spam filters is to throw away anything
where the subject line or from line is ENTIRELY IN CAPITALS.

> Is it possible to overclock a bbc micro, how would you go about doing
> this?

I recall that the Solidisk 4Meg board did this. I think it had it's own
faster memory, as well as a fast 6502 CPU.

Things you need to bear in mind are:
* The on-board RAM won't run faster than 2Mhz
* I/O devices expect to be accessed at 1MHz or 2MHz
* Sideways ROMs that access I/O devices (eg DFS) often have timing loops
  that depend on the CPU running at 2MHz

So, what you would need would be:
* A faster CPU
* A faster clock signal to drive the CPU. The system master clock runs
  16Hz and is divided down to various levels to drive various parts of the
  motherboard. 8MHz and 4MHz are combined to give 6MHz for bits of the
  video. 2MHz and 1MHz go to the CPU, and 1MHz goes to most peripherals.
* Logic to tell the faster CPU to run an normal speed in situations that
  need it. Probably all I/O access (so pick up the signal that stretches
  the CPU clock), and certain sideways ROMs, (so pick up the ROM select
  value).

So, you'd probably set ROM code speeds to
 DFS: normal, BASIC: fast, VIEW: fast.

--
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
jgh@...                - Running on BBCs & Masters with SJ MDFS FileServer
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