158
ATmega128(L)
2467B–09/01
When configured as a Master, the SPI interface has no automatic control of the SS line.
This must be handled by user software before communication can start. When this is
done, writing a byte to the SPI Data Register starts the SPI clock generator, and the
hardware shifts the 8 bits into the Slave. After shifting one byte, the SPI clock generator
stops, setting the end of transmission flag (SPIF). If the SPI interrupt enable bit (SPIE) in
the SPCR register is set, an interrupt is requested. The Master may continue to shift the
next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave
Select, SS
line. The last incoming byte will be kept in the buffer register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated
as long as the SS pin is driven high. In this state, software may update the contents of
the SPI Data Register, SPDR, but the data will not be shifted out by incoming clock
pulses on the SCK pin until the SS
pin is driven low. As one byte has been completely
shifted, the end of transmission flag, SPIF is set. If the SPI interrupt enable bit, SPIE, in
the SPCR register is set, an interrupt is requested. The Slave may continue to place
new data to be sent into SPDR before reading the incoming data. The last incoming byte
will be kept in the buffer register for later use.
Figure 75. SPI Master-Slave Interconnection
The system is single buffered in the transmit direction and double buffered in the receive
direction. This means that bytes to be transmitted cannot be written to the SPI Data
Register before the entire shift cycle is completed. When receiving data, however, a
received character must be read from the SPI Data Register before the next character
has been completely shifted in. Otherwise, the first byte is lost.
In SPI slave mode, the control logic will sample the incoming signal of the SCK pin. To
ensure correct sampling of the clock signal, the frequency of the SPI clock should never
exceed f
osc
/4.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS
pins is
overridden according to
Table 69. For more details on automatic port overrides, refer to
“Alternate Port Functions” on page 66.
Note: 1. See “Alternate Functions of Port B” on page 69 for a detailed description of how to
define the direction of the user defined SPI pins.
Table 69. SPI Pin Overrides
(1)
Pin Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS
User Defined Input
SHIFT
ENABLE
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