Transmit Buffer

The I²C module has a dedicated transmit buffer, I2CxTXB, which is independent from the receive buffer.

The transmit buffer is loaded with an address byte (when ABD = 1), or a data byte, that is copied into the transmit shift register and transmitted onto the bus. When the I2CxTXB register does not contain any transmit data, the Transmit Buffer Empty Status (TXBE) bit is set (TXBE = 1), allowing user software or the DMA to load a new byte into the buffer. When the TXBE bit is set and the I2CxCNT register is nonzero (I2CxCNT != 0), the I²C Transmit Interrupt Flag (I2CxTXIF) bit of the PIR registers is set, and can be used as a DMA trigger. A write to I2CxTXB will clear both the TXBE and I2CxTXIF bits. Setting the Clear Buffer (CLRBF) bit clears I2CxTXIF, the I2Cx Receive Buffer (I2CxRXB) and I2CxTXB.

If user software attempts to load I2CxTXB while it is full, the Transmit Write Error Status (TXWE) bit is set, a NACK is generated, and the new data is ignored. If TXWE is set, user software must clear the bit before attempting to load the buffer again.

When module hardware attempts to transfer the contents of I2CxTXB to the transmit shift register while I2CxTXB is empty (TXBE = 1), the Transmit Underflow Status (TXU) bit is set, I2CxTXB is loaded with 0xFF, and a NACK is generated.

Important: A transmit underflow can only occur when clock stretching is disabled (Clock Stretching Disable (CSD) bit = 1). Clock stretching prevents transmit underflows because the clock is stretched after the 8th falling SCL edge, and is only released upon the write of new data into I2CxTXB.