PIC18F4550ESOSQTP datasheet(202/438 Pages) MICROCHIP

202 / 438 page

PIC18F2455/2550/4455/4550

DS39632E-page 200

19.3.2

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)

The MSSP module consists of a transmit/receive shift

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 (SSPSTAT<0>) 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

(SSPCON1<7>), will be set. User software must clear

the WCOL bit so that it can be determined if the follow-

ing write(s) to the SSPBUF register completed

successfully.

The Buffer Full bit, BF (SSPSTAT<0>), 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. Example 19-1

shows the loading of the SSPBUF (SSPSR) for data

transmission.

The SSPSR is not directly readable or writable and can

only be accessed by addressing the SSPBUF register.

Additionally, the MSSP Status register (SSPSTAT)

indicates the various status conditions.

EXAMPLE 19-1:

LOADING THE SSPBUF (SSPSR) REGISTER

Note:

When the application software is expect-

ing to receive valid data, the SSPBUF

should be read before the next byte of

data to transfer is written to the SSPBUF.

Application software should follow this

process even when the current contents of

SSPBUF are not important.

Note:

The SSPBUF register cannot be used with

read-modify-write instructions, such as

BCF, BTFSC and COMF.

TransmitSPI:

BCF

PIR1, SSPIF

;Make sure interrupt flag is clear (may have been set from previous

transmission).

MOVF

SSPBUF, W

;Perform read, even if the data in SSPBUF is not important

MOVWF

RXDATA

;Save previously received byte in user RAM, if the data is meaningful

MOVF

TXDATA, W

;WREG = Contents of TXDATA (user data to send)

MOVWF

SSPBUF

;Load data to send into transmit buffer

WaitComplete:

;Loop until data has finished transmitting

BTFSS

PIR1, SSPIF

;Interrupt flag set when transmit is complete

BRA

WaitComplete

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