Datasheet

67
4235K–8051–05/08
AT89C51RD2/ED2
Figure 16-4. Data Transmission Format (CPHA = 0)
Figure 16-5. Data Transmission Format (CPHA = 1)
Figure 16-6. CPHA/SS Timing
As shown in Figure 16-4, 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 trans-
mitted (Figure 16-6).
Figure 16-5 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 transmissions (Figure 16-6). This
format may be preferred in systems having only one Master and only one Slave driving the
MISO data line.
16.3.3 Error Conditions
The following flags in the SPSTA signal SPI error conditions:
MSB bit6 bit5 bit4 bit3 bit2 bit1 LSB
bit6 bit5 bit4 bit3 bit2 bit1MSB LSB
1 32 4 5 6 7 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
1 32 4 5 6 7 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)