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'[PIC:] multiple SPI slaves?'
2004\01\25@160253 by Bill Couture

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Hi!

I'm looking at a design for work where we would have one central PIC
playing communications host (probably an 18F452), and 8 "satellite" PICs
controlling hardware (16F818's), communicating via with the host via SPI.

I'm worried, however, about "broadcast messages" to the satellite PICs.
Can the 16F818's listen even if the SS signal is not asserted?  Or can
I assert all of the 16F818's SS lines and have everyone listen?

And, what about the reply to the SPI master?  Do I have to worry about the
"wired-OR" nature of the SDO line, or can I keep a 0 in the SPI transmit
register, and nobody sends a reply?

If the question isn't clear enough, please ask for details.

Thanks,
  Bill

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2004\01\25@161743 by Tom Sefranek

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I have one 18C452 as a master to 27 slaves (also 452s).

Slave select is bogus on the PICs!  (Even the F452s!)
Slave select is an EDGE sync. for input data, and a gate for output data
from the slave.

The slave PICs WILL interrupt even if the SS is NOT active!
(So your proposal to broadcast may actually ACCIDENTLY work!)

Tom


Bill Couture wrote:

{Quote hidden}

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2004\01\25@174452 by Ian McLean

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Yes !  I have also seen this behaviour when trying to use a slave PIC and a
Dallas RTC using SPI (DS1305).  The Slave Select does NOT work properly.
The slave PIC receives and shifts the buffer even when it is not 'selected'.
Resorted to bit-bashing between PIC's.

Rgs
Ian

> {Original Message removed}

2004\01\25@180149 by Thomas C. Sefranek

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I have resorted to fixing the Microchip bug, once again
by using a simple PAL which both qualifies the slave address,
and gates the SPI clock to the slave.

One would think the designers SO close to Motorola
(Who did SPI SS WELL!) would have "inherited" some of the legacy.

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> {Original Message removed}

2004\01\25@184622 by Roy J. Gromlich

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----- Original Message -----
From: "Ian McLean" <EraseMEianmmspam_OUTspamTakeThisOuTOPTUSNET.COM.AU>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Sunday, January 25, 2004 5:41 PM
Subject: Re: [PIC:] multiple SPI slaves?


> Yes !  I have also seen this behaviour when trying to use a slave PIC and
a
> Dallas RTC using SPI (DS1305).  The Slave Select does NOT work properly.
> The slave PIC receives and shifts the buffer even when it is not
'selected'.
> Resorted to bit-bashing between PIC's.
>
> Rgs
> Ian
>
> > {Original Message removed}

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