PIC16(L)F1508/9
DS40001609E-page 174
2011-2015 Microchip Technology Inc.
21.2.2
SPI MODE 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)
To enable the serial port, SSP Enable bit, SSPEN of the
SSPxCON1 register, must be set. To reset or reconfig-
ure SPI mode, clear the SSPEN bit, re-initialize the
SSPxCONx registers and then set the SSPEN bit. This
configures the SDI, SDO, SCK and SS pins as serial
port pins. For the pins to behave as the serial port func-
tion, some must have their data direction bits (in the
TRIS register) appropriately programmed as follows:
• SDIx must have corresponding TRIS bit set
• SDOx must have corresponding TRIS bit cleared
• SCKx (Master mode) must have corresponding
TRIS bit cleared
• SCKx (Slave mode) must have corresponding
TRIS bit set
• SSx must have corresponding TRIS bit set
Any serial port function that is not desired may be
overridden by programming the corresponding data
direction (TRIS) register to the opposite value.
The MSSP 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
eight bits of data have been received, that byte is
moved to the SSPxBUF register. Then, the Buffer Full
Detect bit, BF of the SSPxSTAT register, 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 of the SSPxCON1
register, will be set. User software must clear the
WCOL bit to allow the following write(s) to the
SSPxBUF register to complete 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 of the SSPxSTAT register, indi-
cates 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.
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.
