You've attached code for an SPI MASTER.  I was
asking about an SPI SLAVE.  If you have matching
SPI slave code that works with your bit-banged master
clock pulse I'd like to take a look.

In any case, the answer for me is simple.
I just needed to slow the master clock down enough so
my inefficient C code slave could process each in/out
bit within the clock period.  I did this by
configuring the master to use TMR2 as a timing source.
I was more interested in learning the protocol than
making it fast.

--- Michael Rigby-Jones
<Michael.Rigby-Jones@BOOKHAM.COM> wrote:
> > -----Original Message-----
> > From: Rick Regan [SMTP:rdrdr2k@YAHOO.COM]
> > Sent: Friday, June 27, 2003 4:04 PM
> > To:   PICLIST@MITVMA.MIT.EDU
> > Subject:      Re: [PIC]: SPI clock pulse width
> >
> >   In this case I am writing the slave (on another
> > PIC16F877), so I think I need to know the width.
> > I want to make sure I can detect the pulse by
> polling,
> > and send/receive the data before the next pulse.
> > It looks like in order to do that, I need to set
> the
> > master's clock rate high enough.
> >
> SPI is (usualy) a static bus that can work down to
> DC (although this does
> depend on the slave, check the datasheet). The only
> things you have to worry
> about are the maximum clock speed of your slave, and
> the setup/hold times
> for the clock and data.  Writting a software SPI
> peripheral is really quite
> easy, far simpler than say I2C.  A short snippet
> from a function I wrote a
> while back in C:
>
>         #define SDO LATB0       // use latch
> register on 18 series to avoid
> RMW problems
>         #define SCK LATB1
>         #define SDI RB2
>         #define         SPI_HI  1
>         #define         SPI_LO 0
>
>         uchar ucSpiIn = 0x00;
>
>         SDO = (ucSpiOut & 0x80)? SPI_HI : SPI_LO;
>    // test first bit
> and set SDO
>         NOP();                          // delay to
> allow data to stabilise
> before clocking
>         SCK = SPI_HI;
> // clock high
>         if(SDI) ucSpiIn |= 0x80;
>                    //
> check SDI and set relevant bit in ucSpiIn
>         SCK = SPI_LO;
> // clock low
>
>         SDO = (ucSpiOut & 0x40)? SPI_HI : SPI_LO;
>    // repeat for each
> bit ....
>         NOP();
>         SCK = SPI_HI;
>         if(SDI) ucSpiIn |= 0x40;
>         SCK = SPI_LO;
>
>         ...
>
> It used to be in a loop which I unrolled to get a
> higher speed, purely in
> the interests of saving time rather than any slave
> requirements.  This
> emulates the PIC's MSSP module by reading in a bit
> for each bit output.  If
> you have no requirement for this you can ignore the
> reading part.  This
> should be very easy to translate to assembler:
>
>         #define SDO     PORTB,0
>         #define SCK     PORTB,1
>         #define SDI             PORTB,2
>
>         movlw   0                       ; clear w
>
>         btfsc   SpiOut,7        ; check most
> significant bit and set data
> line accordingly
>         bsf             SDO
>         btfss   SpiOut,7
>         bcf             SDO
>         nop                                     ;
> you will need at least one
> NOP here for reliable operation
>                                                 ; on
> 16 series if  clock and
> data are on the same port.
>         bsf             SCK             ; Set clock
> high.  May need a NOP
> after this for slow
>                                                 ;
> SPI devices or fast PICs,
> check datasheet!
>         btfsc   SDI                     ; check data
> in pin...
>         iorlw   0x80            ; and set relevant
> bit in W
>         bcf             SCK             ; clock low
>
> Repeat for the next 7 bits, of if code space is
> tight put it into a loop.
> For SPI devices that require 16/24 bits simply call
> function multiple times.
>
> Regards
>
> Mike
>
>
>
>
>
>
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