Datasheet
PIC18F97J94 FAMILY
DS30575A-page 358 2012 Microchip Technology Inc.
20.3.6 MASTER MODE
The master can initiate the data transfer at any time
because it controls the SCKx signal. The master deter-
mines when the slave (Processor 2, Figure 20-2) 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). This could be useful in receiver
applications as a “Line Activity Monitor” mode.
The clock polarity is selected by appropriately program-
ming the CKP bit (SSPxCON1<4>). This, then, would
give waveforms for SPI communication, as shown in
Figure 20-3, Figure 20-5 and Figure 20-6, where the
MSB is transmitted first. In Master mode, the SPI clock
rate (bit rate) is user-programmable to be one of the
following:
•F
OSC/4 (or TCY)
•F
OSC/(4 * (SSPxADD + 1)
•FOSC/16 (or 4 • TCY)
•FOSC/64 (or 16 • TCY)
• Timer2 output/2
This allows a maximum data rate (at 64 MHz) of
16.00 Mbps.
Figure 20-3 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 20-3: 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)
Next Q4 Cycle
after Q2
bit 0