169
AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master
clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and receive logic,
and drop any partially received data in the Shift Register.
16.2.2
Master Mode
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. Typically, the
pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven low by
peripheral circuitry when the SPI is configured as a 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 conten-
tion, 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 transmission 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. If the MSTR bit has been cleared by a
slave select, it must be set by the user to re-enable SPI Master mode.
16.2.3
MCU Control Register – MCUCR
Bit 7– SPIPS: SPI Pin Redirection
Thanks to SPIPS (SPI Pin Select) in MCUCR Sfr, SPI pins can be redirected.
On 32 pins packages, SPIPS has the following action:
– When the SPIPS bit is written to zero, the SPI signals are directed on pins MISO,MOSI, SCK and SS.
– When the SPIPS bit is written to one,the SPI signals are directed on alternate SPI pins, MISO_A,
MOSI_A, SCK_A and SS_A.
On 24 pins package, SPIPS has the following action:
– When the SPIPS bit is written to zero, the SPI signals are directed on alternate SPI pins, MISO_A,
MOSI_A, SCK_A and SS_A.
– When the SPIPS bit is written to one,the SPI signals are directed on pins MISO,MOSI, SCK and SS.
Note that programming port are always located on alternate SPI port.
16.2.4
SPI Control Register – SPCR
Bit 7 – SPIE: SPI Interrupt Enable
This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if the Global Inter-
rupt Enable bit in SREG is set.
Bit
7
65432
10
SPIPS
–
PUD
–
IVSEL
IVCE
MCUCR
Read/Write
R/W
R
R/W
R
R/W
Initial Value
0
00000
00
Bit
76543210
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
SPCR
Read/Write
R/W
Initial Value
00000000