
72
AT89C51RB2/RC2
4180E–8051–10/06
Figure 28. Data Transmission Format (CPHA = 0)
Figure 29. Data Transmission Format (CPHA = 1)
Figure 30. CPHA/SS Timing
As shown in
Figure 28, 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
Figure 29 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 30). This format may be preffered 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
bit1
MSB
LSB
13
2
4
567
8
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
13
2
4
567
8
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)