XC886/888CLM
Serial Interfaces
User’s Manual
12-33
V1.3, 2010-02
Serial Interfaces, V 1.0
The Data Width Selection supports the transfer of frames of any data length, from 2-bit
“characters” up to 8-bit “characters”. Starting with the LSB (CON.HB = 0) allows
communication with SSC devices in synchronous mode or with serial interfaces such as
the one in 8051. Starting with the MSB (CON.HB = 1) allows operation compatible with
the SPI interface.
Regardless of the data width selected and whether the MSB or the LSB is transmitted
first, the transfer data is always right-aligned in registers TB and RB, with the LSB of the
transfer data in bit 0 of these registers. The data bits are rearranged for transfer by the
internal shift register logic. The unselected bits of TB are ignored; the unselected bits of
RB will not be valid and should be ignored by the receiver service routine.
The Clock Control allows the transmit and receive behavior of the SSC to be adapted to
a variety of serial interfaces. A specific shift clock edge (rising or falling) is used to shift
out transmit data, while the other shift clock edge is used to latch in receive data. Bit
CON.PH selects the leading edge or the trailing edge for each function. Bit CON.PO
selects the level of the shift clock line in the idle state. Thus, for an idle-high clock, the
leading edge is a falling one, a 1 - to - 0 transition (see
Figure 12-12 Serial Clock Phase and Polarity Options
When initializing the devices for serial communication, one device must be selected for
master operation while all other devices must be programmed for slave operation.
12.3.1.2
Full-Duplex Operation
The various devices are connected through three lines. The definition of these lines is
always determined by the master: the line connected to the master’s data output line
Shift Clock
MS_CLK/SS_CLK
CON.
PH
CON.
PO
1
0
0
0
1
1
1
0
Pins
MTSR/MRST
Transmit Data
Shift Data
Latch Data
First
Bit
Last
Bit
*
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