AT89C51ID2-RDTIM datasheet(102/159 Pages) ATMEL

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102

AT89C51ID2

4289A–8051–09/03

Figure 39. Full-Duplex Master-Slave Interconnection

Master mode

The SPI operates in Master mode when the Master bit, MSTR

(1), in the SPCON register

is set. Only one Master SPI device can initiate transmissions. Software begins the trans-

mission from a Master SPI module by writing to the Serial Peripheral Data Register

(SPDAT). If the shift register is empty, the byte is immediately transferred to the shift

SCK. Simultaneously, another byte shifts in from the Slave on the Master’s MISO pin.

The transmission ends when the Serial Peripheral transfer data flag, SPIF, in SPSTA

becomes set. At the same time that SPIF becomes set, the received byte from the Slave

is transferred to the receive data register in SPDAT. Software clears SPIF by reading

the Serial Peripheral Status register (SPSTA) with the SPIF bit set, and then reading the

SPDAT.

Slave mode

The SPI operates in Slave mode when the Master bit, MSTR

(2), in the SPCON register is

cleared. Before a data transmission occurs, the Slave Select pin, SS, of the Slave

device must be set to’0’. SS must remain low until the transmission is complete.

In a Slave SPI module, data enters the shift register under the control of the SCK from

the Master SPI module. After a byte enters the shift register, it is immediately transferred

to the receive data register in SPDAT, and the SPIF bit is set. To prevent an overflow

condition, Slave software must then read the SPDAT before another byte enters the

shift register

(3). A Slave SPI must complete the write to the SPDAT (shift register) at

least one bus cycle before the Master SPI starts a transmission. If the write to the data

transmission.

Transmission Formats

Software can select any of four combinations of serial clock (SCK) phase and polarity

using two bits in the SPCON: the Clock POLarity (CPOL

(4)) and the Clock PHAse

(CPHA

4). CPOL defines the default SCK line level in idle state. It has no significant

effect on the transmission format. CPHA defines the edges on which the input data are

sampled and the edges on which the output data are shifted (Figure 40 and Figure 41).

The clock phase and polarity should be identical for the Master SPI device and the com-

municating Slave device.

8-bit Shift register

SPI

Clock Generator

Master MCU

8-bit Shift register

MISO

MOSI

SCK

VSS

VDD

Slave MCU

The SPI module should be configured as a Master before it is enabled (SPEN set). Also

the Master SPI should be configured before the Slave SPI.

The SPI module should be configured as a Slave before it is enabled (SPEN set).

The maximum frequency of the SCK for an SPI configured as a Slave is the bus clock

speed.

Before writing to the CPOL and CPHA bits, the SPI should be disabled (SPEN =’0’).

AT89C51ID2-RDTIM Datasheet(PDF) 102 Page - ATMEL Corporation

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