SPMCK16Z4MPV20 datasheet(250/500 Pages) FREESCALE

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MULTICHANNEL COMMUNICATION INTERFACE

M68HC16 Z SERIES

10-10

USER’S MANUAL

10.3.4.2 CPHA = 1 Transfer Format

Figure 10-4 is a timing diagram of an 8-bit, MSB-first SPI transfer in which CPHA

equals one. Two waveforms are shown for SCK, one for CPOL equal to zero and an-

other for CPOL equal to one. The diagram may be interpreted as a master or slave

timing diagram since the SCK, MISO and MOSI pins are directly connected between

the master and the slave. The MISO signal shown is the output from the slave and the

MOSI signal shown is the output from the master. The SS line is the slave select input

to the slave.

Figure 10-4 CPHA = 1 SPI Transfer Format

For a master, writing to the SPDR initiates the transfer. For a slave, the first edge of

SCK indicates the start of a transfer. The SPI is left-shifted on the first and each suc-

ceeding odd clock edge, and data is latched on the second and succeeding even clock

edges.

SCK is inactive for the last half of the eighth SCK cycle. For a master, SPIF is set at

the end of the eighth SCK cycle (after the seventeenth SCK edge). Since the last SCK

edge occurs in the middle of the eighth SCK cycle, however, the slave has no way of

knowing when the end of the last SCK cycle occurs. The slave therefore considers the

transfer complete after the last bit of serial data has been sampled, which corresponds

to the middle of the eighth SCK cycle.

When CPHA is one, the SS line may remain at its active low level between transfers.

This format is sometimes preferred in systems having a single fixed master and only

one slave that needs to drive the MISO data line.

12345678

SCK CYCLE #

(FOR REFERENCE)

SCK (CPOL = 0)

SCK (CPOL = 1)

MOSI

(FROM MASTER)

MSB

54321

LSB

MSB

54321

LSB

MISO

(FROM SLAVE)

SS (TO SLAVE)

CPHA = 1 SPI TRANSFER

* NOT DEFINED BUT NORMALLY LSB OF PREVIOUSLY TRANSMITTED CHARACTER

Freescale Semiconductor, Inc.

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