Thanks for the example....this is very helpful.

Thanks,

- Ted


-----Original Message-----
From: Michael Rigby-Jones [mailto:Michael.Rigby-Jones@BOOKHAM.COM]
Sent: Thursday, April 15, 2004 9:36 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC:] I2C & SPI on the same PIC...

>-----Original Message-----
>From: Ted Larson [mailto:ted@LARSONLAND.COM]
>Sent: 15 April 2004 16:10
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [PIC:] I2C & SPI on the same PIC...
>
>
>All this talk about SPI devices, has me thinking about an interesting
>problem I have, that perhaps someone can suggest an easy solution for.
>
>I am not too familiar with SPI, I am just learning about it, however, I
>am pretty familiar with I2C at this point.  I have a circuit that has a
>PIC18F252 as an I2C master, and it talks to several I2C slave devices.
>I have recently been presented with an SPI device that I need to
>interface to the same I2C master PIC.  The SSP hardware on the PIC is
>pretty much I2C or SPI, and not both.  Short of switching everything to
>SPI, what are my options?  Is there some kind of I2C to SPI converter
>out there?  I have poked around, and there doesn't appear to be a PIC
>with more than one SSP on it...perhaps there is one I am missing?
>
>Thanks,
>
>- Ted

SPI is an order of magnitude more simple than I2C.  Think of a shift
register that has a chip select signal and that's about as complex as it
gets.

The good news is that SPI is also much easier than I2C to bit bash, so
unless you need very high speed, you can keep the MSSP for I2C master and
bit bash the SPI:

Set CS (chip select) low
Set SDO (data out) according to bit (most significant bit first) Set SCK
(clock) high Read SDI (data in) Set SCK low

In assembly this boils down to relatively few instructions, so it's often
practical to code this inline for all 8 bits rather than use a loop for
optimal speed.

#define CS LATB,0
#define SDO LATB,1
#define SDI PORTB,2
#define SCK LATB,3

clrf     DataIn
bsf    CS        ; chip select low

; clock in/out bit 7
btfss  DataOut,7
bcf    SDO       ; data out low
btfsc  DataOut,7
bcf    SDO       ; data out high

bsf    SCK       ; clock high

btfsc  SDI       ; if input high
bsf    DataIn, 7 ; set bit in DataIn

bcf    SCK       ; clock low

; clock in/out bit 6
btfss  DataOut,6
bcf    SDO       ; data out low
btfsc  DataOut,6
bcf    SDO       ; data out high

bsf    SCK       ; clock high

btfsc  SDI       ; if input high
bsf    DataIn, 6 ; set bit in DataIn

bcf    SCK       ; clock low
...etc....

This emulates the operation of the MSSP, where 8 bits are read in at the
same time that 8 bits are transmitted (i.e. to read only, you would perform
a dummy write).  If you only ever need to read or write separately you can
make a separate read and write function that would execute more quickly.

There was a recent discussion on the faster method of clocking out 8 bits in
the manner, regarding some benchmakrs for the Maxim "MAXQ" microcontroller.
You should be able to find this in the archive.

Regards

Mike




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