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PIC18FXX39
DS30485A-page 130
Preliminary
2002 Microchip Technology Inc.
16.3.5
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 16-2) 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 program-
ming the CKP bit (SSPCON1<4>). This then, would
give waveforms for SPI communication as shown in
Figure 16-3, Figure 16-5, and Figure 16-6, 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)
This allows a maximum data rate (at 40 MHz) of
10.00 Mbps.
Figure 16-3 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.
FIGURE 16-3:
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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