34.6.2.5.1 Receiving Address Packets (SCLSM = 0)

When CTRLA.SCLSM = 0, the I²C client stretches the SCL line according to Figure 34-9. When the I²C client is properly configured, it waits for a Start condition.

When a Start condition is detected, the successive address packet is received and checked by the address match logic. If the received address is not a match, the packet is rejected, and the I²C client waits for a new Start condition. If the received address is a match, the Address Match bit in the Interrupt Flag register (INTFLAG.AMATCH) is set.

SCL is stretched until the I²C client clears INTFLAG.AMATCH. As the I²C client holds the clock by forcing SCL low, the software has unlimited time to respond.

The direction of a transaction is determined by reading the Read/Write Direction bit in the Status register (STATUS.DIR). This bit is updated only when a valid address packet is received.

If the Transmit Collision bit in the Status register (STATUS.COLL) is set, this indicates that the last packet addressed to the I²C client had a packet collision. A collision causes the SDA and SCL lines to be released without any notification to software. Therefore, the next AMATCH interrupt is the first indication of the previous packet’s collision. Collisions are intended to follow the SMBus Address Resolution Protocol (ARP).

After the address packet is received from the I²C host, one of two cases arises based on transfer direction.

Case 1: Address packet accepted – Read flag set

The STATUS.DIR bit is ‘1’, indicating an I²C host read operation. The SCL line is forced low, stretching the bus clock. If an ACK is sent, I²C client hardware will set the Data Ready bit in the Interrupt Flag register (INTFLAG.DRDY), indicating data are needed for transmit. If a NACK is sent, the I²C client will wait for a new Start condition and address match.

Typically, software immediately acknowledges the address packet by sending an ACK/NACK bit. The I²C client Command bit field in the Control B register (CTRLB.CMD) can be written to 0x3 for both read and write operations, as the command execution is dependent on the STATUS.DIR bit. Writing ‘1’ to INTFLAG.AMATCH will also cause an ACK/NACK to be sent corresponding to the CTRLB.ACKACT bit.

Case 2: Address packet accepted – Write flag set

The STATUS.DIR bit is cleared, indicating an I²C host write operation. The SCL line is forced low, stretching the bus clock. If an ACK is sent, the I²C client waits for data to be received. Data, repeated start or stop can be received.

If a NACK is sent, the I²C client waits for a new Start condition and address match. Typically, software immediately acknowledges the address packet by sending an ACK/NACK. The I²C client command CTRLB.CMD = 3 can be used for both read and write operations, as the command execution is dependent on STATUS.DIR.

Writing ‘1’ to INTFLAG.AMATCH also causes an ACK/NACK to be sent corresponding to the CTRLB.ACKACT bit.