CC1110Fx / CC1111Fx
therefore not safe to use. Instead, the
assertion of the UxCSR.TX_BYTEbit should be
used as an indication on when new data can
be written to UxDBUF. For DMA transfers this
is handled automatically, but with the limitation
that the UxGDR.CPHA bit must be set to zero.
the framing error bit, UxCSR.FE, will be
asserted. This means that this bit is updated 1
bit duration later than the 2 other above
mentioned bits. The UxCSR.ACTIVEbit will be
de-asserted after the second stop bit (if
UxUCR.SPB=1).
For
systems
requiring
setting
UxGDR.CPHA=1,the DMA can not be used.
13.14.2 SPI Mode
This section describes the SPI mode of
operation for synchronous communication. In
SPI mode, the USART communicates with an
external system through a 3-wire or 4-wire
interface. The interface consists of the pins
MOSI, MISO, SCK and SSN. Refer to section
13.4 on page 91 for I/O configuration.
Also note that the USARTx TX complete
interrupt occurs approximately 1 byte period
prior to the USARTx RX complete interrupt.
In SPI master mode, only the MOSI, MISO,
and SCK should be configured as peripherals
(see section 13.4.6.1 and 13.4.6.2). If the
external slave requires a slave select signal
(SSN) this can be implemented by using a
general-purpose I/O pin and control from SW.
The SPI mode includes the following features:
• 3-wire (master) and 4-wire SPI interface
• Master and slave modes
• Configurable SCK polarity and phase
• Configurable LSB or MSB first transfer
13.14.2.2 SPI Slave Operation
An SPI byte transfer in slave mode is
controlled by the external system. The data on
the MOSI input is shifted into the receive
register controlled by the serial clock SCK,
which is an input in slave mode. At the same
time the byte in the transmit register is shifted
out onto the MISO output.
The SPI mode is selected when UxCSR.MODE
is set to 0.
In SPI mode, the USART can be configured to
operate either as an SPI master or as an SPI
slave by setting UxCSR.SLAVE to 0 or 1,
recpectively.
The UxCSR.ACTIVE bit goes high when the
transfer starts and low when the transfer ends.
When the transfer ends, the UxCSR.RX_BYTE
bit is set to 1
13.14.2.1 SPI Master Operation
An SPI byte transfer in master mode is initiated
when the UxDBUF register is written. The
USART generates the SCK signal using the
baud rate generator (see section 13.14.3) and
shifts the provided byte from the transmit
register onto the MOSI output. At the same
time the receive register shifts in the received
byte from the MISO input pin.
At the end of the transfer, the USARTx RX
complete CPU interrupt flag, TCON.URXxIF, is
asserted and the received data byte is
available in UxDBUF. An interrupt request is
generated if IEN0.URXxIE=1. The USARTx
TX
complete
CPU
interrupt
flag,
IRCON2.UTXxIF, is asserted at the start of
the operation and an interrupt request is
generated if IEN2.UTXxIE=1.
The polarity and clock phase of the serial clock
SCK is selected by UxGCR.CPOL and
UxGCR.CPHA. The order of the byte transfer is
selected by the UxGCR.ORDERbit.
The expected polarity and clock phase of SCK
is selected by UxGCR.CPOLand UxGCR.CPHA
as shown in Figure 40. The expected order of
the byte transfer is selected by the
UxGCR.ORDER bit.
The UxCSR.ACTIVE bit goes high when the
transfer starts and low when the transfer ends.
When the transfer ends, the UxCSR.TX_BYTE
bit is set to 1.
13.14.2.3 Slave Select pin (SSN)
At the end of the transfer, the USARTx RX
complete CPU interrupt flag, TCON.URXxIF, is
asserted and the received data byte is
available in UxDBUF. An interrupt request is
generated if IEN0.URXxIE=1
When the USART is operating in SPI slave
mode, a 4-wire interface is used with the Slave
Select (SSN) pin as an input to the SPI (edge
controlled). The SPI slave becomes active
after a falling edge on SSN and will receive
data on the MOSI input and send data on the
MISO output. After a rising edge on SSN, the
SPI slave is inactive and will not receive data.
Note that the MISO output is not tri-stated
Since UxDBUF
is double-buffered, the
assertion of the USARTx TX complete CPU
interrupt flag (IRCON2.UTXxIF) happens just
after a transmission has been initiated, and is
SWRS033E
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