ST6285B datasheet(53/78 Pages) STMICROELECTRONICS

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ST62T85B/E85B

4.6 SERIAL PERIPHERAL INTERFACE (SPI)

The on-chip SPI is an optimized serial synchro-

nous interface that supports a wide range 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

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 (I²C-bus, etc).

When data is serialized, the MSB is the first 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. With these 3 lines, the SPI can operate in the

following operating modes: Software SPI, S-BUS,

I²C-bus and as a standard serial I/O (clock, data,

enable). An interrupt request can be generated af-

ter eight clock pulses. Figure 28 shows the SPI

block diagram.

The SCL line clocks, on the falling edge, the shift

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.

In master mode, SCL is programmed as output, a

clock signal must be generated by software to set

and reset the port line.

Figure 28. 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

DIN

D0............................D7

to Processor Data Bus

OPR Reg.

DIN

SCL

Sin

Sout

SPI Interrupt Disable Register

SPI Data Register

Data Reg

Direction

Data Reg

Direction

DOUT

Write

Read

Interrupt

VR01504

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