I'm triyng to use the SSP Module's I2C implementation. It seems like a
lot of the hardware should be implemented within the SSP module - but
Microchip doesn't provide much help in documentation.

Daniel Najemy - Numaliine Power Systems

> -----Original Message-----
> From: Keith Howell [SMTP:keithh@ARCAM.CO.UK]
> Sent: Wednesday, January 14, 1998 9:41 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Which Port For I2C?
>
> > [snip lot of text about using bsf and bsf for twiddling SDA and SCL]
>
> No, it really is okay to use bsf and bcf to twiddle SDA and SCL.
> I do.
> I twiddle the bits in the TRIS registers.
> AFAIK this does not disturb other bits in the register, or I/O bits.
> So I2C events do not disturb other I/O bits.
>
> The disturbances discussed previously only happen if you twiddle
> the PORT register bits.
> Which you should not be doing, because
> the I2C signals are open collector!
> You should be twiddling the TRIS bits,
> and keeping the corresponding PORT bits low.
>
> You have to avoid other I/O bits disturbing the I2C PORT bits.
>
> BTW, the last chap's I2C file looks flaky for a start.
> Look at the code:
> > SendI2CStart
> >       movlw   ~(SDA_MASK|SCL_MASK)   ; make sure output
> >                                       ; lines are low
> >       andwf   I2CPORT                 ; when outputs are enabled
>
> There's nothing to guarantee the timing.
> The I2C spec says the start condition is SCL falling when SDA is low.
> AND the time between SDA falling and SCL falling must be
> at least 4 microseconds! I use something like this:
>
> I2C_SCL_HI      MACRO
>         bsf     TRISC, I2C_SCL          ; Set SCL high
>         ENDM
> I2C_SCL_LO      MACRO
>         bcf     TRISC, I2C_SCL          ; Set SCL low
>         ENDM
> I2C_SDA_HI      MACRO
>         bsf     TRISC, I2C_SDA          ; Set SDA high
>         ENDM
> I2C_SDA_LO      MACRO
>         bcf     TRISC, I2C_SDA          ; Set SDA low
>         ENDM
> doWait3microseconds     MACRO
>         nop
>         nop
>         nop
>         ENDM
>
>
> So that you do a START like so:
>
>         page1                           ; meaning data page
>         I2C_SDA_LO                      ; SDA goes low during SCL high
>         doWait3microseconds             ;
>         I2C_SCL_LO                      ; Start clock train
>         page0                           ; return with data page == 0