WebIDE Pi I2C "Error accessing..."

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If talking to an AVR from the Raspberry Pi and you see that error message, read this:
http://elinux.org/BCM2835_datasheet_errata#p35_I2C_clock_stretching

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See...

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Below...

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Even though the problem is actually with the Pi I2C hardware, it can be worked around. This is a changed to the slave CPU, not the Pi. Putting a time delay in the Wire library twi.c allows talking to Arduino-likes from the Raspberry Pi on I2C. My atmega328p CPU is @ 8 MHz so perhaps this needs to be tweaked for 16 MHz cases (normal Arduino). Anyway, this bandaid completely stopped the problem (twi.c, in SIGNAL(TWI_vect) routine):

Code: Select all

    // Slave Receiver
    case TW_SR_SLA_ACK:   // addressed, returned ack
    case TW_SR_GCALL_ACK: // addressed generally, returned ack
      delayMicroseconds(3);