AT89C5122 ATMEL [ATMEL Corporation], AT89C5122 Datasheet - Page 141

• Manufacturer Part Number: AT89C5122
• Description: MICROCONTROLLER WITH USB AND SMART CARD READER INTERFACES
• Manufacturer: ATMEL [ATMEL Corporation]

4202D–SCR–06/05
MOSI (from Master)
SCK Cycle Number
MISO (from Slave)
SCK (CPOL = 0)
SCK (CPOL = 1)
MOSI (from Master)
SPEN (internal)
MISO (from Slave)
SCK Cycle Number
SCK (CPOL = 0)
SCK (CPOL = 1)
Capture point
SS (to Slave)
SPEN (internal)
Capture Point
SS (to Slave)
Figure 86. Data Transmission Format (CPHA = 0)
Figure 87. Data Transmission Format (CPHA = 1)
As shown in Figure 86, 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 88).
Figure 88. CPHA/SS Timing
Figure 87 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-
MISO/MOSI
(CPHA = 0)
(CPHA = 1)
Master SS
Slave SS
Slave SS
MSB
MSB
MSB
1
MSB
1
Byte 1
2
bit6
bit6
2
bit6
bit6
3
bit5
bit5
3
bit5
bit5
Byte 2
bit4
4
bit4
bit4
bit4
4
bit3
bit3
5
bit3
bit3
5
Byte 3
6
bit2
bit2
6
bit2
bit2
7
bit1
bit1
7
bit1
bit1
LSB
8
LSB
LSB
8
LSB
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