HC705C9AGRSSLASHD datasheet(102/157 Pages) NXP

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General Release Speciﬁcation

SPI Signal Description

MC68HC705C9A — Rev. 2.0

Serial Peripheral Interface

10.4.3 Serial Clock (SCK)

The master clock is used to synchronize data movement both in and out

of the device through its MOSI and MISO lines. The master and slave

devices are capable of exchanging a byte of information during a

sequence of eight clock cycles. Since SCK is generated by the master

device, this line becomes an input on a slave device.

As shown in Figure 10-1, four possible timing relationships may be

chosen by using control bits CPOL and CPHA in the serial peripheral

control register (SPCR). Both master and slave devices must operate

with the same timing. The master device always places data on the

MOSI line a half cycle before the clock edge (SCK), in order for the slave

device to latch the data.

Two bits (SPR0 and SPR1) in the SPCR of the master device select the

clock rate. In a slave device, SPR0 and SPR1 have no effect on the

operation of the SPI.

10.4.4 Slave Select (SS)

The slave select (SS) input line is used to select a slave device. It has to

be low prior to data transactions and must stay low for the duration of the

transaction.The SS line on the master must be tied high. In master

mode, if the SS pin is pulled low during a transmission, a mode fault error

flag (MODF) is set in the SPSR. In master mode the SS pin can be

selected to be a general-purpose output (when configured as an

MC68HC05C9A) by writing a one in bit 5 of the port D data direction

When CPHA = 0, the shift clock is the OR of SS with SCK. In this clock

phase mode, SS must go high between successive characters in an SPI

message. When CPHA = 1, SS may be left low for several SPI

characters. In cases where there is only one SPI slave MCU, its SS line

could be tied to VSS as long as CPHA = 1 clock modes are used.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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