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Subject: Re: Getting rid of that damn noise.
From: Andrew Jamieson (ajamiesn_at_mira.net)
Date: 2001-12-28


> Andrew wrote...
> If it is the LCD serial data, then the ultimate cause is a lack of proper
> power-supply de-coupling at the CPU. This is quite a common thing, as
> complete de-coupling is virtually impossible without seperate supplies ...
>
> Rob replies...
> Are you saying it is possibly the power rails being effected by the output
> fets that drive the display?

Well, if by FETs you mean the internal port drivers of the SH-1; then yes,
that is exactly what I mean.

Since my last post, I've done some digging (you should be used to me posting
before doing my research by now ... :)

Internal Osc of the LCD: 15 - 19Khz (17 typical)

Frame Frequency of LCD: 57.7 - 73 Hz (65.4 at 17Khz typical Osc
freq)

The oscillator (and thus the frame frequency) are active at all times the
LCD _display_ is on (you can turn off the display without removing power
from the LCD), and so it is unlikely it is this that is causing the
interference if the noise disappears just because the LCD is not being
accessed by the CPU. The culprit is definitely the power rail, IMHO, and
the likely candidates for this are the SMPS (Switch Mode Power Supply), and
CPU coupling.

It's interesting that the interference is not volume sensitive; therefore it
is probably being caused in the amplifier stage (not the pre-amp which
changes the input signal to adjust the volume), from what I know of the MAS
chip used (and once again, shooting from the hip without research ...) these
stages are both contained on chip. However, there are quite possibly
seperate supplies for them (or at least seperate grounds), and changing the
coupling on this supply would fix the problem too .....

I suppose I should look at the MAS data sheet.

CM



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