Datasheet

PIC18F97J94 FAMILY
DS30575A-page 356  2012 Microchip Technology Inc.
20.3.2 OPERATION
When initializing the SPI, several options need to be
specified. This is done by programming the appropriate
control bits (SSPxCON1<5:0> and SSPxSTAT<7:6>).
These control bits allow the following to be specified:
• I/O pins must be mapped to the SPI peripheral in
order to function. See Section 11.15 “PPS-Lite”
for an explanation of the PPS-Lite mapping
feature.
• Master mode (SCKx is the clock output)
• Slave mode (SCKx is the clock input)
• Clock Polarity (Idle state of SCKx)
• Data Input Sample Phase (middle or end of data
output time)
• Clock Edge (output data on rising/falling edge of
SCKx)
• Clock Rate (Master mode only)
• Slave Select mode (Slave mode only)
Each MSSPx module consists of a Transmit/Receive
Shift register (SSPxSR) and a Buffer register
(SSPxBUF). The SSPxSR shifts the data in and out of
the device, MSb first. The SSPxBUF holds the data that
was written to the SSPxSR until the received data is
ready. Once the 8 bits of data have been received, that
byte is moved to the SSPxBUF register. Then, the
Buffer Full detect bit, BF (SSPxSTAT<0>), and the
interrupt flag bit, SSPxIF, are set. This double-buffering
of the received data (SSPxBUF) allows the next byte to
start reception before reading the data that was just
received. Any write to the SSPxBUF register during
transmission/reception of data will be ignored and the
Write Collision Detect bit, WCOL (SSPxCON1<7>), will
be set. User software must clear the WCOL bit so that
it can be determined if the following write(s) to the
SSPxBUF register completed successfully.
When the application software is expecting to receive
valid data, the SSPxBUF should be read before the
next byte of data to transfer is written to the SSPxBUF.
The Buffer Full bit, BF (SSPxSTAT<0>), indicates when
SSPxBUF has been loaded with the received data
(transmission is complete). When the SSPxBUF is
read, the BF bit is cleared. This data may be irrelevant
if the SPI is only a transmitter. Generally, the MSSPx
interrupt is used to determine when the transmission/
reception has completed. If the interrupt method is not
going to be used, then software polling can be done to
ensure that a write collision does not occur.
Example 20-1 shows the loading of the SSPxBUF
(SSPxSR) for data transmission.
The SSPxSR is not directly readable or writable and
can only be accessed by addressing the SSPxBUF
register. Additionally, the SSPxSTAT register indicates
the various status conditions.
20.3.3 OPEN-DRAIN OUTPUT OPTION
The drivers for the SDOx output and SCKx clock pins
can be optionally configured as open-drain outputs.
This feature allows the voltage level on the pin to be
pulled to a higher level through an external pull-up
resistor, and allows the output to communicate with
external circuits without the need for additional level
shifters. For more information, see Section 11.1.3
“Open-Drain Outputs”.
The open-drain output option is controlled by the
SSPxOD bits (ODCON1<1:0>). Setting an SSPxOD bit
configures the SDOx and SCKx pins for the
corresponding module for open-drain operation.
EXAMPLE 20-1: LOADING THE SSP1BUF (SSP1SR) REGISTER
Note: To avoid lost data in Master mode, a
read of the SSPxBUF must be per-
formed to clear the Buffer Full (BF)
detect bit (SSPxSTAT<0>) between
each transmission.
LOOP BTFSS SSP1STAT, BF ;Has data been received (transmit complete)?
BRA LOOP ;No
MOVF SSP1BUF, W ;WREG reg = contents of SSP1BUF
MOVWF RXDATA ;Save in user RAM, if data is meaningful
MOVF TXDATA, W ;W reg = contents of TXDATA
MOVWF SSP1BUF ;New data to xmit