The USCI module provides automatic clock activation for use with low-power modes. When the USCI clock source is inactive because the device is in a low-power mode, the USCI module automatically activates it when needed, regardless of the control-bit settings for the clock source. The clock remains active until the USCI module returns to its idle condition. After the USCI module returns to the idle condition, control of the clock source reverts to the settings of its control bits. In I2C slave mode, no internal clock source is required because the clock is provided by the external master. It is possible to operate the USCI in I2C slave mode while the device is in LPM4 and all internal clock sources are disabled. The receive or transmit interrupts can wake up the CPU from any low-power mode.

USCI Interrupts in I2C Mode

The USCI has only one interrupt vector that is shared for transmission, reception, and state change. USCI_Ax and USC_Bx do not share the same interrupt vector. Each interrupt flag has its own interrupt enable bit. When an interrupt is enabled and the GIE bit is set, the interrupt flag generates an interrupt request. DMA transfers are controlled by the UCTXIFG and UCRXIFG flags on devices with a DMA controller.

I2C Transmit Interrupt Operation

The UCTXIFG interrupt flag is set by the transmitter to indicate that UCBxTXBUF is ready to accept another character. An interrupt request is generated if UCTXIE and GIE are also set. UCTXIFG is automatically reset if a character is written to UCBxTXBUF or if a NACK is received. UCTXIFG is set when UCSWRST = 1 and the I2C mode is selected. UCTXIE is reset after a PUC or when UCSWRST = 1.

I2C Receive Interrupt Operation

The UCRXIFG interrupt flag is set when a character is received and loaded into UCBxRXBUF. An interrupt request is generated if UCRXIE and GIE are also set. UCRXIFG and UCRXIE are reset after a PUC signal or when UCSWRST = 1. UCRXIFG is automatically reset when UCxRXBUF is read.

I2C State Change Interrupt Operation

UCBxIV, Interrupt Vector Generator

The USCI interrupt flags are prioritized and combined to source a single interrupt vector. The interrupt vector register UCBxIV is used to determine which flag requested an interrupt. The highest-priority enabled interrupt generates a number in the UCBxIV register that can be evaluated or added to the PC to automatically enter the appropriate software routine. Disabled interrupts do not affect the UCBxIV value. Any access, read or write, of the UCBxIV register automatically resets the highest-pending interrupt flag. If another interrupt flag is set, another interrupt is immediately generated after servicing the initial interrupt