From: Jim Ruxton <jim.ruxton@gmail.com>
To: "piclist@mit.edu" <piclist@mit.edu>
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Date: Mon, 15 Mar 2021 01:41:44 -0700
Subject: } Re: [PIC] 1 Controller and 18 Peripheral PICs
Thread-Topic: } Re: [PIC] 1 Controller and 18 Peripheral PICs
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Curious Harold if you typically put a ground plane under you clock lines=20
and size the trace appropriately when using series source termination as=20
described below in order to create a 50 ohm microstrip line or do you=20
just add the series resistor? Would you use something around 33 ohms as=20
the resistor?
> One thing to watch out for in any SPI or shift register system is clock
> bounce due to transmission line effects. If a single clock line is bused
> past several chips, it is really hard to get rid of reflections. My best
> solution has been to use a buffer chip (like maybe a 245 or similar with
> all inputs tied together at the clock source) and have a separate series
> source terminated clock line going out to each device. Series source
> termination is really clever. When the clock goes high, it only goes to
> 1/2 voltage after the series termination since the transmission line look=
s
> like infinite length with its characteristic impedance matching the serie=
s
> termination resistor. This forms a voltage divider dividing the pulse hig=
h
> voltage by 2. That half voltage step travels down the transmission line
> and hits the open end at the chip (chip input is high impedance). The
> voltage immediately doubles to the full clock voltage due to the lack of
> termination at the destination. The step to full voltage then travels bac=
k
> towards the source and eventually the voltage at the series termination
> resistor jumps up to full voltage. Series source termination with single
> destinations has worked well for me.

Thanks,

Jim

--=20
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