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Sheet目录3881

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PIC18F6585/8585/6680/8680

DS30491C-page 192

2004 Microchip Technology Inc.

17.3.2

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 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. 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 com-

pleted. 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 17-1 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 17-1:

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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