PIC18F4550ESOSQTP datasheet(204/438 Pages) MICROCHIP

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PIC18F2455/2550/4455/4550

DS39632E-page 202

19.3.5

MASTER MODE

The master can initiate the data transfer at any time

because it controls the SCK. The master determines

when the slave (Processor 2, Figure 19-2) is to

broadcast data by the software protocol.

In Master mode, the data is transmitted/received as

soon as the SSPBUF register is written to. If the SPI is

only going to receive, the SDO output could be dis-

abled (programmed as an input). The SSPSR register

will continue to shift in the signal present on the SDI pin

at the programmed clock rate. As each byte is

received, it will be loaded into the SSPBUF register as

if a normal received byte (interrupts and status bits

appropriately set). This could be useful in receiver

applications as a “Line Activity Monitor” mode.

The clock polarity is selected by appropriately

programming the CKP bit (SSPCON1<4>). This, then,

would give waveforms for SPI communication as

shown in Figure 19-3, Figure 19-5 and Figure 19-6,

where the MSB is transmitted first. In Master mode, the

SPI clock rate (bit rate) is user-programmable to be one

of the following:

•FOSC/4 (or TCY)

•FOSC/16 (or 4 • TCY)

•FOSC/64 (or 16 • TCY)

• Timer2 output/2

This allows a maximum data rate (at 48 MHz) of

12.00 Mbps.

When used in Timer2 Output/2 mode, the bit rate can

be configured using the PR2 Period register and the

Timer2 prescaler. However, writing to SSPBUF does

not clear the current TMR2 value in hardware. Depend-

ing upon the current value of TMR2 when the user firm-

ware writes to SSPBUF, this can result in an

unpredictable MSb bit width, unless the procedure of

Example 19-2 is used.

Figure 19-3 shows the waveforms for Master mode.

When the CKE bit is set, the SDO data is valid before

there is a clock edge on SCK. The change of the input

sample is shown based on the state of the SMP bit. The

time when the SSPBUF is loaded with the received

data is shown.

EXAMPLE 19-2:

LOADING SSPBUF WITH THE TIMER2/2 CLOCK MODE

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

BCF

T2CON, TMR2ON

;Turn off timer when loading SSPBUF

CLRF

TMR2

;Set timer to a known state

MOVF

TXDATA, W

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

MOVWF

SSPBUF

;Load data to send into transmit buffer

BSF

T2CON, TMR2ON

;Start timer to begin transmission

WaitComplete:

;Loop until data has finished transmitting

BTFSS

PIR1, SSPIF

;Interrupt flag set when transmit is complete

BRA

WaitComplete

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