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USART Operation: SPI Mode
15-4 USART Peripheral Interface, SPI Mode
15.2 USART Operation: SPI Mode
In SPI mode, serial data is transmitted and received by multiple devices using
a shared clock provided by the master. An additional pin, STE, is provided as
to enable a device to receive and transmit data and is controlled by the master.
Three or four signals are used for SPI data exchange:
- SIMO Slave in, master out
Master mode: SIMO is the data output line.
Slave mode: SIMO is the data input line.
- SOMI Slave out, master in
Master mode: SOMI is the data input line.
Slave mode: SOMI is the data output line.
- UCLK USART SPI clock
Master mode: UCLK is an output.
Slave mode: UCLK is an input.
- STE Slave transmit enable. Used in 4-pin mode to allow multiple
masters on a single bus. Not used in 3-pin mode.
4-Pin master mode:
When STE is high, SIMO and UCLK operate normally.
When STE is low, SIMO and UCLK are set to the input direction.
4-pin slave mode:
When STE is high, RX/TX operation of the slave is disabled and
SOMI is forced to the input direction.
When STE is low, RX/TX operation of the slave is enabled and
SOMI operates normally.
15.2.1 USART Initialization and Reset
The USART is reset by a PUC or by the SWRST bit. After a PUC, the SWRST
bit is automatically set, keeping the USART in a reset condition. When set, the
SWRST bit resets the URXIEx, UTXIEx, URXIFGx, OE, and FE bits and sets
the UTXIFGx flag. The USPIEx bit is not altered by SWRST. Clearing SWRST
releases the USART for operation. See also chapter USART Module, I2C
mode for USART0 when reconfiguring from I
2
C mode to SPI mode.
Note: Initializing or Re-Configuring the USART Module
The required USART initialization/re-configuration process is:
1) Set SWRST (BIS.B #SWRST,&UxCTL)
2) Initialize all USART registers with SWRST=1 (including UxCTL)
3) Enable USART module via the MEx SFRs (USPIEx)
4) Clear SWRST via software (BIC.B #SWRST,&UxCTL)
5) Enable interrupts (optional) via the IEx SFRs (URXIEx and/or UTXIEx)
Failure to follow this process may result in unpredictable USART behavior.