C8051F70X_0910 Datasheet, PDF (226/290 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F70x/71x
Master
Device MISO
MOSI
SCK
Slave
MISO Device
MOSI
SCK
Figure 29.3. 3-Wire Single Master and 3-Wire Single Slave Mode Connection
Diagram
Master
Device
GPIO
MISO
MOSI
SCK
NSS
MISO
MOSI
SCK
NSS
Slave
Device
MISO
MOSI
SCK
NSS
Slave
Device
Figure 29.4. 4-Wire Single Master Mode and 4-Wire Slave Mode Connection
Diagram
29.3. SPI0 Slave Mode Operation
When SPI0 is enabled and not configured as a master, it will operate as a SPI slave. As a slave, bytes are
shifted in through the MOSI pin and out through the MISO pin by a master device controlling the SCK sig-
nal. A bit counter in the SPI0 logic counts SCK edges. When 8 bits have been shifted through the shift reg-
ister, the SPIF flag is set to logic 1, and the byte is copied into the receive buffer. Data is read from the
receive buffer by reading SPI0DAT. A slave device cannot initiate transfers. Data to be transferred to the
master device is pre-loaded into the shift register by writing to SPI0DAT. Writes to SPI0DAT are double-
buffered, and are placed in the transmit buffer first. If the shift register is empty, the contents of the transmit
buffer will immediately be transferred into the shift register. When the shift register already contains data,
the SPI will load the shift register with the transmit buffer’s contents after the last SCK edge of the next (or
current) SPI transfer.
When configured as a slave, SPI0 can be configured for 4-wire or 3-wire operation. The default, 4-wire
slave mode, is active when NSSMD1 (SPI0CN.3) = 0 and NSSMD0 (SPI0CN.2) = 1. In 4-wire mode, the
NSS signal is routed to a port pin and configured as a digital input. SPI0 is enabled when NSS is logic 0,
and disabled when NSS is logic 1. The bit counter is reset on a falling edge of NSS. The NSS signal must
be driven low at least 2 system clocks before the first active edge of SCK for each byte transfer.
Figure 29.4 shows a connection diagram between two slave devices in 4-wire slave mode and a master
device.
226
Rev. 0.3