SLAZ374AH October   2012  – August 2021 MSP430FR5721

 

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6.37 USCI41

USCI Module

Category

Functional

Function

UCBUSY bit of eUSCIA module might not work reliable when device is in SPI mode.

Description

When eUSCIA is configured in SPI mode, the UCBUSY bit might get stuck to 1 or start toggling after transmission is completed. This happens in all four combinations of Clock Phase and Clock Polarity options (UCAxCTLW0.UCCKPH & UCAxCTLW0.UCCKPL bits) as well as in Master and Slave mode. There is no data loss or corruption. However the UCBUSY cannot be used in its intended function to check if transmission is completed.  Because the UCBUSY bit is stuck to 1 or toggles, the clock request stays enabled and this adds additional current consumption in low power mode operation.

Workaround

For correct functional implementation check on transmit or receive interrupt flag UCTXIFG/UCRXIFG instead of UCBUSY to know if the UCAxTXBUF buffer is empty or ready for the next complete character.
To reduce the additional current it is recommended to either reset the SPI module (UCAxCTLW0.UCSWRST) in the UCBxCTLW0  or send a dummy byte 0x00 after the intended SPI transmission is completed.