PIC18F4515T-I/PT【PDF 62/234 页】IC MCU FLASH 24KX16 44TQFP

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PIC18F2X1X/4X1X

DS39636D-page 156

2009 Microchip Technology Inc.

16.3.2

OPERATION

When initializing SPI operation, 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 consists of a Transmit/Receive Shift regis-

ter (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 8 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.

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 (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 interface is only a transmitter. Generally, the MSSP

interrupt is used to determine when the transmission/

reception has completed. The SSPBUF must be read

and/or written. 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 16-1 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 16-1:

LOADING THE SSPBUF (SSPSR) REGISTER

LOOP

BTFSS

SSPSTAT, BF

;Has data been received (transmit complete)?

BRA

LOOP

;No

MOVF

SSPBUF, W

;WREG reg = contents of SSPBUF

MOVWF

RXDATA

;Save in user RAM, if data is meaningful

MOVF

TXDATA, W

;W reg = contents of TXDATA

MOVWF

SSPBUF

;New data to xmit

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