> -----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





=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================
Any questions about Bookham's E-Mail service should be directed to postmaster@bookham.com.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics