PIC18F47J53 Datasheet, PDF (295/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

PIC18F47J53 FAMILY
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:
• 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 MSSP 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 (BF) detect
bit (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
or 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.
Note:
When the application software is expecting
to receive valid data, the SSPxBUF should
be read before the next byte of transfer
data is written to the SSPxBUF. Application
software should follow this process even
when the current contents of SSPxBUF
are not important.
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 MSSP 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 provides 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, provided the SDOx or SCKx pin is not multi-
plexed with an ANx analog function. This allows the
output to communicate with external circuits without the
need for additional level shifters. For more information,
see Section 10.1.4 “Open-Drain Outputs”.
The open-drain output option is controlled by the
SPI2OD and SPI1OD bits (ODCON3<1:0>). Setting an
SPIxOD bit configures both the SDOx and SCKx pins for
the corresponding open-drain operation.
EXAMPLE 20-1: LOADING THE SSP1BUF (SSP1SR) REGISTER
LOOP
BTFSS
BRA
MOVF
SSP1STAT, BF
LOOP
SSP1BUF, W
;Has data been received (transmit complete)?
;No
;WREG reg = contents of SSP1BUF
MOVWF RXDATA
;Save in user RAM, if data is meaningful
MOVF TXDATA, W
MOVWF SSP1BUF
;W reg = contents of TXDATA
;New data to xmit
 2010 Microchip Technology Inc.
Preliminary
DS39964B-page 295