AT89C51AC3-RDTIM datasheet(86/140 Pages) ATMEL

86 / 140 page

AT89C51AC3

4383D–8051–02/08

When the Master device transmits data to the Slave device via the MOSI line, the Slave

device responds by sending data to the Master device via the MISO line. This implies

full-duplex transmission with both data out and data in synchronized with the same clock

(Figure 45).

Figure 45. 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 SPSCR

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 (SPSCR) 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 trans-

ferred 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 reg-

ister) at least one bus cycle before the Master SPI starts a transmission. If the write to

the data register is late, the SPI transmits the data already in the shift register from the

previous 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

(CPHA4). 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 46 and Figure 47).

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’).

AT89C51AC3-RDTIM 数据表(PDF) 86 Page - ATMEL Corporation

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