PIC16F1938_13 Datasheet, PDF (240/488 Pages) Microchip Technology – 28/40/44-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver

PIC16(L)F1938/9
24.2.2 SPI MODE OPERATION
When initializing the SPI, several options need to be
specified. This is done by programming the appropriate
control bits (SSPCON1<5:0> and SSPSTAT<7:6>).
These control bits allow the following to be specified:
• Master mode (SCK is the clock output)
• Slave mode (SCK is the clock input)
• Clock Polarity (Idle state of SCK)
• Data Input Sample Phase (middle or end of data
output time)
• Clock Edge (output data on rising/falling edge of
SCK)
• Clock Rate (Master mode only)
• Slave Select mode (Slave mode only)
To enable the serial port, SSP Enable bit, SSPEN of the
SSPCON1 register, must be set. To reset or reconfig-
ure SPI mode, clear the SSPEN bit, re-initialize the
SSPCONx 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:
• SDI must have corresponding TRIS bit set
• SDO must have corresponding TRIS bit cleared
• SCK (Master mode) must have corresponding
TRIS bit cleared
• SCK (Slave mode) must have corresponding
TRIS bit set
• SS 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
(SSPSR) and a buffer register (SSPBUF). The SSPSR
shifts the data in and out of the device, MSb first. The
SSPBUF holds the data that was written to the SSPSR
until the received data is ready. Once the eight bits of
data have been received, that byte is moved to the
SSPBUF register. Then, the Buffer Full Detect bit, BF
of the SSPSTAT register, and the interrupt flag bit,
SSPIF, are set. This double-buffering of the received
data (SSPBUF) allows the next byte to start reception
before reading the data that was just received. Any
write to the SSPBUF register during
transmission/reception of data will be ignored and the
write collision detect bit WCOL of the SSPCON1
register, will be set. User software must clear the
WCOL bit to allow the following write(s) to the SSPBUF
register to complete successfully.
When the application software is expecting to receive
valid data, the SSPBUF should be read before the next
byte of data to transfer is written to the SSPBUF. The
Buffer Full bit, BF of the SSPSTAT register, indicates
when SSPBUF has been loaded with the received data
(transmission is complete). When the SSPBUF 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 SSPSR is not directly readable or writable and can
only be accessed by addressing the SSPBUF register.
Additionally, the SSPSTAT register indicates the
various Status conditions.
FIGURE 24-5:
SPI MASTER/SLAVE CONNECTION
SPI Master SSPM<3:0> = 00xx
= 1010 _SDO
Serial Input Buffer
(BUF)
SPI Slave SSPM<3:0> = 010x
SDI
Serial Input Buffer
(SSPBUF)
Shift Register
(SSPSR)
MSb
LSb
SDI
SCK
Serial Clock
SDO
SCK
Shift Register
(SSPSR)
MSb
LSb
Slave Select
General I/O
SS
Processor 1
(optional)
Processor 2
DS40001574C-page 240
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