PIC16 datasheet(186/418 Pages) MICROCHIP

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PIC16(L)F1508/9

DS40001609C-page 186

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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

reconfigure 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

function, 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

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

the various Status conditions.

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