Manual

108
AT89C5131
4136A–USB–03/03
Figure 45. Data Transmission Format (CPHA = 0)
Figure 46. Data Transmission Format (CPHA = 1)
Figure 47. CPHA/SS
Timing
As shown in Figure 46, the first SCK edge is the MSB capture strobe. Therefore the
Slave must begin driving its data before the first SCK edge, and a falling edge on the SS
pin is used to start the transmission. The SS pin must be toggled high and then low
between each byte transmitted (Figure 43).
Figure 47 shows an SPI transmission in which CPHA is’1’. In this case, the Master
begins driving its MOSI pin on the first SCK edge. Therefore the Slave uses the first
SCK edge as a start transmission signal. The SS
pin can remain low between transmis-
sions (Figure 42). This format may be preferable in systems having only one Master and
only one Slave driving the MISO data line.
MSB bit6 bit5 bit4 bit3 bit2 bit1 LSB
bit6 bit5 bit4 bit3 bit2 bit1MSB
LSB
13245678
Capture point
SS (to Slave)
MISO (from Slave)
MOSI (from Master)
SCK (CPOL = 1)
SCK (CPOL = 0)
SPEN (internal)
SCK cycle number
MSB bit6 bit5 bit4 bit3 bit2 bit1 LSB
bit6 bit5 bit4 bit3 bit2 bit1
MSB LSB
132 45678
Capture point
SS (to Slave)
MISO (from Slave)
MOSI (from Master)
SCK (CPOL = 1)
SCK (CPOL = 0)
SPEN (internal)
SCK cycle number
Byte 1 Byte 2
Byte 3
MISO/MOSI
Master SS
Slave SS
(CPHA = 1)
Slave SS
(CPHA = 0)