MC68HC05C8A Datasheet, PDF (60/116 Pages) Motorola, Inc – Microcontrollers

Serial Peripheral Interface (SPI)
10.3.3 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 one-half cycle before the
clock edge (SCK), so the slave device can 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 SPI operation.
SS
SCK
SCK
SCK
SCK
MISO/MOSI
MSB
6
5
4
3
2
1
0
INTERNAL STROBE FOR DATA CAPTURE (ALL MODES)
Figure 10-1. Data Clock Timing Diagram
10.3.4 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. If it goes low, a mode fault error flag (MODF) is set in the
SPSR.
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 V_SS as long as
CPHA = 1 clock modes are used.
MC68HC05C8A • MC68HCL05C8A • MC68HSC05C8A Data Sheet, Rev. 5.1
60
Freescale Semiconductor