Rainbow Electronics ATmega103L - Manual (Page 56)

ATmega603/103
56
Figure 38. SPI Master-Slave Interconnection
The system is single buffered in the transmit direction and double buffered in the receive direction. This means that charac-
ters to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed. When receiving
data, however, a received byte must be read from the SPI Data Register before the next byte has been completely shifted
in. Otherwise, the first byte is lost.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK and SS pins is overridden according to the following
table:
Note: See “Alternate Functions of Port B” on page 78 f or a detailed description and how to define the direction of the user
defined SPI pins.
SS Pin Functionality
When the SPI is configured as a master (MSTR in SPCR is set), the user can determine the direction of the SS pin. If SS is
configured as an output, the pin is a general output pin which does not affect the SPI system. If SS is configured as an
input, it must be hold high to ensure Master SPI operation. If the SS pin is driven low by peripheral circuitry when the SPI is
configured as master with the SS pin defined as an input, the SPI system interprets this as another master selecting the
SPI as a slave and starting to send data to it. To avoid bus contention, the SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a slave. As a result of the SPI becoming a slave, the
MOSI and SCK pins become inputs.
2. The SPIF flag in SPSR is set, and if the SPI interrupt is enabled and the I-bit in SREG is set, the interrupt routine will
be executed.
Thus, when interrupt-driven SPI transmittal is used in master mode, and there exists a possibility that SS is driven low, the
interrupt should always check that the MSTR bit is still set. Once the MSTR bit has been cleared by a slave select, it must
be set by the user to re-enable SPI master mode.
When the SPI is configured as a slave, the SS pin is always input. When SS is held low, the SPI is activated and MISO
becomes an output if configured so by the user. All other pins are inputs. When SS is driven high, all pins are inputs, and
the SPI is passive, which means that it will not receive incoming data. Note that the SPI logic will be reset once the SS pin
is brought high. If the SS pin is brought high during a transmission, the SPI will stop sending and receiving immediately and
both data received and data sent must be considered as lost.
Table 23. SPI Pin Overrides
PIN Direction, Master SPI Direction, Slave SPI
MOSI User Defined Input
MISO Input User Defined
SCK User Defined Input
SS User Defined Input
8 BIT SHIFT REGISTER
MASTER
MISO
SPI
CLOCK GENERATOR
8 BIT SHIFT REGISTER
SLAVE
MISO
MOSI MOSI
SCK SCK
SS SS
V
CC
MSB LSB MSB LSB