USCI Operation: SPI Mode
20-12
Universal Serial Communication Interface, SPI Mode
20.3.7 Using the SPI Mode with Low Power Modes
The USCI module provides automatic clock activation for SMCLK for use with
low-power modes. When SMCLK is the USCI clock source, and 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. Automatic clock activation
is not provided for ACLK.
When the USCI module activates an inactive clock source, the clock source
becomes active for the whole device and any peripheral configured to use the
clock source may be affected. For example, a timer using SMCLK will
increment while the USCI module forces SMCLK active.
In SPI 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 SPI slave
mode while the device is in LPM4 and all clock sources are disabled. The
receive or transmit interrupt can wake up the CPU from any low power mode.
20.3.8 SPI Interrupts
The USCI has one interrupt vector for transmission and one interrupt vector
for reception.
SPI Transmit Interrupt Operation
The UCxTXIFG interrupt flag is set by the transmitter to indicate that
UCxTXBUF is ready to accept another character. An interrupt request is
generated if UCxTXIE and GIE are also set. UCxTXIFG is automatically reset
if a character is written to UCxTXBUF. UCxTXIFG is set after a PUC or when
UCSWRST = 1. UCxTXIE is reset after a PUC or when UCSWRST = 1.
Note:
Writing to UCxTXBUF in SPI Mode
Data written to UCxTXBUF when UCxTXIFG = 0 may result in erroneous
data transmission.
SPI Receive Interrupt Operation
The UCxRXIFG interrupt flag is set each time a character is received and
loaded into UCxRXBUF. An interrupt request is generated if UCxRXIE and GIE
are also set. UCxRXIFG and UCxRXIE are reset by a system reset PUC signal
or when UCSWRST = 1. UCxRXIFG is automatically reset when UCxRXBUF
is read.
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