Rockbox mail archiveSubject: Re: Getting rid of that damn noise.
Re: Getting rid of that damn noise.
From: Rob & Liz Ward <wards_at_paradise.net.nz>
Date: Fri, 28 Dec 2001 23:37:30 +1300
Ok. I have finally seen the schematic. One comment - the MC34063 chip (U15)
on the i/f and power sheet is a switch-mode power supply chip from Motorola
(an old trashy chip in my opinion - there are much nicer ones than this
part). It looks as if they are using it in a buck configuration (step
down) - acting purely as a battery charger (6V?). The short between pins 7 &
8 mean there is no current limiting for the switcher.
With SMPSU's, layout is very important, and as such, the components D9, C16,
and battery negatives should all be drawn in star config to a common point
at the chips gnd pin. (I know this is picky, but the drawing becomes much
easier to read if the parts are grouped into a std config).
Ok, a few more comments...
D7 simply stops backfeeds from battery to Sw. reg.
Two fets T1 and T2 look like high-side switches with T2 feeding into a 3.3V
switchy regulator (Vcc). I think this is a National Semiconductor Simple
Switcher (TM) buck converter (nice).
D3,4,5,6 look like the backlight leds...
Hope that helps a bit.
----- Original Message -----
From: "Andrew Jamieson" <ajamiesn_at_mira.net>
Sent: Friday, December 28, 2001 9:17 PM
Subject: Re: Getting rid of that damn noise.
> > Andrew wrote...
> > If it is the LCD serial data, then the ultimate cause is a lack of
> > 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
> > 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
> 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
> 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),
> CPU coupling.
> It's interesting that the interference is not volume sensitive; therefore
> 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
> chip used (and once again, shooting from the hip without research ...)
> stages are both contained on chip. However, there are quite possibly
> seperate supplies for them (or at least seperate grounds), and changing
> coupling on this supply would fix the problem too .....
> I suppose I should look at the MAS data sheet.
Received on 2001-12-28