PIC17C752-08SLASHCL datasheet(137/328 Pages) MICROCHIP

137 / 328 page

DS30289A-page 137

PIC17C7XX

15.1.4

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 15-8) is to broad-

cast 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 disabled

(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 program-

ming bit CKP (SSPCON1<4>). This then would give

waveforms for SPI communication as shown in

Figure 15-9, Figure 15-11, and Figure 15-12 where the

MSb is transmitted ﬁrst. In master mode, the SPI clock

rate (bit rate) is user programmable to be one of the fol-

lowing:

•FOSC/4 (or TCY)

•FOSC/16 (or 4 • TCY)

•FOSC/64 (or 16 • TCY)

• Timer2 output/2

This allows a maximum bit clock frequency (at 33 MHz)

of 8.25 MHz.

Figure 15-9 shows the waveforms for master mode.

When CKE = 1, 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.

FIGURE 15-9: SPI MODE WAVEFORM (MASTER MODE)

SCK

(CKP = 0

SCK

(CKP = 1

SCK

(CKP = 0

SCK

(CKP = 1

4 clock

modes

Input

Sample

Input

Sample

SDI

bit7

bit0

SDO

bit7

bit6

bit5

bit4

bit3

bit2

bit1

bit0

bit7

bit0

SDI

SSPIF

(SMP = 1)

(SMP = 0)

(SMP = 1)

CKE = 1)

CKE = 0)

CKE = 1)

CKE = 0)

(SMP = 0)

Write to

SSPBUF

SSPSR to

SSPBUF

SDO

bit7

bit6

bit5

bit4

bit3

bit2

bit1

bit0

(CKE = 0)

(CKE = 1)

Next Q4 cycle

after Q2

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