7-bit Transmission with Address Hold Enabled

Setting the AHEN bit enables additional clock stretching and interrupt generation after the eighth falling edge of a received matching address. Once a matching address has been clocked in, CKP is cleared and the SSPxIF interrupt is set.

Figure 1 displays a standard waveform of a 7-bit address slave transmission with AHEN enabled.

  1. 1.Bus starts Idle.
  2. 2.Master sends Start condition; the S bit is set; SSPxIF is set if interrupt on Start detect is enabled.
  3. 3.Master sends matching address with R/W bit set. After the eighth falling edge of the SCL line the CKP bit is cleared and SSPxIF interrupt is generated.
  4. 4.Slave software clears SSPxIF.
  5. 5.Slave software reads the ACKTIM, R/W and D/A bits to determine the source of the interrupt.
  6. 6.Slave reads the address value from the SSPxBUF register clearing the BF bit.
  7. 7.Slave software decides from this information if it wishes to ACK or not ACK and sets the ACKDT bit accordingly.
  8. 8.Slave sets the CKP bit releasing SCL.
  9. 9.Master clocks in the ACK value from the slave.
  10. 10.Slave hardware automatically clears the CKP bit and sets SSPxIF after the ACK if the R/W bit is set.
  11. 11.Slave software clears SSPxIF.
  12. 12.Slave loads value to transmit to the master into SSPxBUF setting the BF bit.
    Important: SSPxBUF cannot be loaded until after the ACK.
  13. 13.Slave sets the CKP bit releasing the clock.
  14. 14.Master clocks out the data from the slave and sends an ACK value on the ninth SCL pulse.
  15. 15.Slave hardware copies the ACK value into the ACKSTAT bit.
  16. 16.Steps 10-15 are repeated for each byte transmitted to the master from the slave.
  17. 17.If the master sends a not ACK the slave releases the bus allowing the master to send a Stop and end the communication.
    Important: Master must send a not ACK on the last byte to ensure that the slave releases the SCL line to receive a Stop.
Figure 1. I2C Slave, 7-bit Address, Transmission (AHEN = 1)