INTEDG datasheet(249/478 Pages) MICROCHIP

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DS41414D-page 249

PIC16(L)F1946/47

24.2.3

SPI MASTER MODE

The master can initiate the data transfer at any time

because it controls the SCKx line. The master

determines when the slave (Processor 2, Figure 24-5)

is to broadcast data by the software protocol.

In Master mode, the data is transmitted/received as

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

is only going to receive, the SDOx output could be dis-

abled (programmed as an input). The SSPxSR register

will continue to shift in the signal present on the SDIx

pin at the programmed clock rate. As each byte is

received, it will be loaded into the SSPxBUF register as

if a normal received byte (interrupts and Status bits

appropriately set).

The clock polarity is selected by appropriately

programming the CKP bit of the SSPxCON1 register

and the CKE bit of the SSPxSTAT register. This then,

would give waveforms for SPI communication as

shown in Figure 24-6, Figure 24-8 and Figure 24-9,

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

• Fosc/(4 * (SSPxADD + 1))

Figure 24-6 shows the waveforms for Master mode.

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

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

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

time when the SSPxBUF is loaded with the received

data is shown.

FIGURE 24-6:

SPI MODE WAVEFORM (MASTER MODE)

SCKx

(CKP = 0

SCKx

(CKP = 1

SCKx

(CKP = 0

SCKx

(CKP = 1

4 Clock

Modes

Input

Sample

Input

Sample

SDIx

bit 7

bit 0

SDOx

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

bit 7

SDIx

SSPxIF

(SMP = 1)

(SMP = 0)

(SMP = 1)

CKE = 1)

CKE = 0)

CKE = 1)

CKE = 0)

(SMP = 0)

Write to

SSPxBUF

SSPxSR to

SSPxBUF

SDOx

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

(CKE = 0)

(CKE = 1)

bit 0

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