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The SMBus must accommodate any mixture of devices that support Packet Error Checking and devices that do not. A device that acts as a slave and supports the PEC must always be prepared to perform the slave transfer with or without a PEC, verify the correctness of the PEC if present, and only process the message if the PEC is correct. Implementations are encouraged to issue a NACK if the PEC is present but not correct.
5.4.1.3. Slave implementation A slave device that implements Packet Error Checking must be prepared to receive and transmit data with or without a PEC. During a slave receive transfer, after the device has identified the protocol and command must accept and check the additional PEC for validity. During a slave transmit transfer, the slave transmitter must respond to additional clocks after the last byte transfer and furnish a PEC to the master receiver requesting it. Each bus transaction requires a Packet Error Code (PEC) calculation by both the transmitter and receiver within each packet. The PEC uses an 8-bit cyclic redundancy check (CRC-8) of each read or write bus transaction to calculate a Packet Error Code (PEC). The PEC may be calculated in any way that conforms to a CRC-8 represented by the polynomial, C(x) = x8 + x2 + x1 + 1 and must be calculated in the order of the bits as received. Calculating the PEC for transmission or reception is implemented in a method chosen by the device manufacturer. It is possible to perform the check with low-cost hardware or firmware algorithm that could process the message bit-by-bit or with a byte-wise look-up table. The SMBus web page provides some example CRC-8 methods. The PEC is appended to the message as dictated by the protocols in section 5.5. The PEC calculation includes all bytes in the transmission, including address, command and data. The PEC calculation does not include ACK, NACK, START, STOP nor Repeated START bits. This means that the PEC is computed over the entire message from the first START condition. Whether a device implements packet error checking may be determined by the specification revision code that is present in the SpecificationInfo() command for a Smart Battery, Smart Battery Charger or Smart Battery Selector. See these individual specifications for exact revision coding identities. It may also be discovered in the UDID, defined in section 5.6.1, for other devices.
