PIC18CXX8
DS30475A-page 144
Advanced Information
2000 Microchip Technology Inc.
15.3.4
SLAVE MODE
In Slave mode, the data is transmitted and received as
the external clock pulses appear on SCK. When the
last bit is latched, the SSPIF interrupt flag bit is set.
While in Slave mode, the external clock is supplied by
the external clock source on the SCK pin. This external
clock must meet the minimum high and low times, as
specified in the electrical specifications.
While in SLEEP mode, the slave can transmit/receive
data. When a byte is received, the device will wake-up
from SLEEP.
15.3.5
SLAVE SELECT SYNCHRONIZATION
The SS pin allows a Synchronous Slave mode. The
SPI must be in Slave mode with SS pin control
enabled (SSPCON1<3:0> = 04h). The pin must not
be driven low for the SS pin to function as an input.
The Data Latch must be high. When the SS pin is
low, transmission and reception are enabled and
the SDO pin is driven. When the SS pin goes high,
the SDO pin is no longer driven, even if in the mid-
dle of a transmitted byte, and becomes a floating
output. External pull-up/pull-down resistors may be
desirable, depending on the application.
When the SPI module resets, the bit counter is forced
to 0. This can be done by either forcing the SS pin to a
high level, or clearing the SSPEN bit.
To emulate two-wire communication, the SDO pin can
be connected to the SDI pin. When the SPI needs to
operate as a receiver, the SDO pin can be configured
as an input. This disables transmissions from the SDO.
The SDI can always be left as an input (SDI function),
since it cannot create a bus conflict.
FIGURE 15-3: SLAVE SYNCHRONIZATION WAVEFORM
Note 1: When the SPI is in Slave mode with SS pin
control enabled, (SSPCON<3:0> = 0100)
the SPI module will reset if the SS pin is
set to VDD.
2: If the SPI is used in Slave mode with CKE
set, then the SS pin control must be
enabled.
SCK
(CKP = 1
SCK
(CKP = 0
Input
Sample
SDI
bit7
SDO
bit7
bit6
bit7
SSPIF
(SMP = 0)
CKE = 0)
(SMP = 0)
Write to
SSPBUF
SSPSR to
SSPBUF
SS
bit0
bit7
bit0
Next Q4 Cycle
after Q2
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