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ST62T28C/E28C
4.6 SERIAL PERIPHERAL INTERFACE (SPI)
The on-chip SPI is an optimized serial synchro-
nous interfacethat supports a widerange of indus-
try standard SPI specifications. The on-chip SPI is
controlled by small and simple user software to
perform serial data exchange. The serial shift
clock can be implemented either by software (us-
ing the bit-set and bit-reset instructions), with the
on-chip Timer 1 by externally connecting the SPI
clock pin to the timer pin or by directly applying an
external clock to the Scl line.
The peripheral is composed by an 8-bit Data/shift
Register and a 4-bit binary counter while the Sin
pin is the serial shift input and Sout is the serial
shift output. These two lines can be tied together
to implement two wires protocols (I C-bus, etc).
When data is serialized, the MSB is thefirst bit. Sin
has to be programmed as input. For serial output
operation Sout has to be programmed as open-
drain output.
The SCL,Sin and Sout SPI clock and data signals
are connected to 3 I/O lines on the same external
pins. Withthese 3 lines, theSPI can operate in the
following operating modes: Software SPI, S-BUS,
I C-bus and as a standard serial I/O (clock, data,
enable). An interrupt request can be generated af-
ter eight clock pulses. Figure 34 shows the SPI
block diagram.
The SCL line clocks, on the falling edge, the shift
register and the counter. To allow SPI operation in
slave mode, the SCL pin must be programmed as
input and an external clock must be supplied to
this pin to drive the SPI peripheral.
WARNING
: In all cases, in both slave and master
mode, the SCL pin must always be configured as
input (Reset state).
In master mode, the SCL signal must be generat-
ed by software and output on another free I/O port
pin which must beconnected externally to the SCL
input pin.
Figure 34. SPI Block Diagram
Set Res
CLK
RESET
4-Bit Counter
(Q4=High after Clock8)
Data Reg
Direction
I/O Port
8-Bit Data
Shift Register
Reset
Load
DOUT
Output
Enable
8-Bit Tristate Data I/O
RESET
I/O Port
I/O Port
CP
CP
DIN
D0............................D7
to Processor Data Bus
Q4
Q4
OPR Reg.
DIN
SCL
Sin
Sout
SPI Interrupt Disable Register
SPI Data Register
Data Reg
Direction
Data Reg
Direction
DOUT
Write
Read
M0
1
Interrupt
VR01504