36.3.12 Data Byte Count

The data byte count refers to the number of data bytes in a complete I²C packet. The data byte count does not include address bytes. The I²C Byte Count (I2CxCNT) register is used to specify the length, in bytes, of the complete transaction. The value loaded into I2CxCNT will be decremented by module hardware each time a data byte is transmitted or received by the module.

Important: The I2CxCNT register will not decrement past a zero value.

When a byte transfer causes the I2CxCNT register to decrement to ‘0’, the Byte Count Interrupt Flag (CNTIF) bit is set, and if the Byte Count Interrupt Enable (CNTIE) is set, the general purpose I²C Interrupt Flag (I2CxIF) bit of the Peripheral Interrupt Registers (PIR) is also set. If the I²C Interrupt Enable (I2CxIE) bit of the Peripheral Interrupt Enable (PIE) registers is set, module hardware will generate an interrupt event.

Important: The I2CxIF bit is read-only and can only be cleared by clearing all the interrupt flag bits of the I2CxPIR register.

The I2CxCNT register can be read at any time, but it is recommended that a double read is performed to ensure a valid count value.

The I2CxCNT register can be written to; however, care is required to prevent register corruption. If the I2CxCNT register is written to during the 8th falling SCL edge of a reception, or during the 9th falling SCL edge of a transmission, the register value may be corrupted. In Client mode, I2CxCNT can be safely written to any time the clock is not being stretched (CSTR = 0) or after a Stop condition has been received (Stop Condition Interrupt Flag (PCIF) = 1). In Host mode, I2CxCNT can be safely written to any time the Host Data Ready (MDR) or Bus Free (BFRE) bits are set. If the I²C packet is longer than 255 bytes, the I2CxCNT register can be updated mid-message to prevent the count from reaching zero; however, the preventative measures listed above must be followed.

When in either Client Read or Host Write mode and the I2CxCNT value is non-zero (I2CxCNT != 0), the value of the ACKDT bit is used as the acknowledgement response. When I2CxCNT reaches zero (I2CxCNT = 0), the value of the Acknowledge End of Count (ACKCNT) bit is used for the acknowledgment response.

In Host read or write operations, when the I2CxCNT register is clear (I2CxCNT = 0) and the Restart Enable (RSEN) bit is clear, host hardware automatically generates a Stop condition upon the 9th falling edge of SCL. When I2CxCNT is clear (I2CxCNT = 0) and RSEN is set (RSEN = 1), host hardware will stretch the clock while it waits for the Start (S) bit to be set (S = 1). When the Start bit has been set, module hardware transmits a Restart condition followed by the address of the client it wishes to communicate with.