SPI Bit Banging

micahp

Has anyone been succesful with bit banging SPI on the stamp? I need simultaneous 2 way communication on MISO and MOSI so SHIFTIN/SHIFTOUT won't work for me. Here is what I've tried so far:

Spi:

    recv_byte = 0
    LOW pgm_sck

    FOR i = 0 TO  7

        ' set MSB to MOSI pin
        p = send_byte & $80
        IF p <> 0 THEN
            HIGH pgm_mosi
        ELSE
            LOW pgm_mosi
        ENDIF

        ' shift to next bit
        send_byte = send_byte << 1    

        ' receive data
        recv_byte = recv_byte << 1
        p = IN10
        IF p = 1 THEN
            recv_byte = recv_byte + 1
        ENDIF

        PULSOUT pgm_sck, spi_pulse_length ' ??? what should the pulse length be ???

        'HIGH pgm_sck
        'PAUSE 2 ' ??? how long
        'LOW pgm_sck
        'PAUSE 2 ' ??? how long

    NEXT
    RETURN

Bruce Bates

micahp -

The PBASIC Stamp is single-threaded, and executes one instruction at a time, so simultaneous anything just can not be done. On the other hand, that may not be necessary.

What device are you using that claims that simultaneous read/write access is required? In my experience, it's usually an option rather than a requirement, and in that case MISO and MOSI can often be connected together. This sometimes requires a resistor or a diode, but this can be the circumvention.

Regards,

Bruce Bates

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micahp

Somebody said...

What device are you using that claims that simultaneous read/write access is required? In my experience, it's usually an option rather than a requirement, and in that case MISO and MOSI can often be connected together. This sometimes requires a resistor or a diode, but this can be the circumvention.

I am attempting to make a serial AVR ISP programmer that simulate the AVR910 protocol. It requires simultaneous MISO/MOSI. And yes I am acutely aware of the BS2 limitations -- figured if the clock pulsing where slow enough it would be possible.