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C8051F380/1/2/3/4/5/6/7/C
Master
Device
GPIO
MISO
MOSI
SCK
NSS
MISO
MOSI
SCK
NSS
MISO
MOSI
Slave
Device
Slave
Device
SCK
NSS
Figure 25.4. 4-Wire Single Master Mode and 4-Wire Slave Mode Connection Diagram
25.3. 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. Note that the NSS sig-
nal must be driven low at least 2 system clocks before the first active edge of SCK for each byte transfer.
Figure 25.4 shows a connection diagram between two slave devices in 4-wire slave mode and a master
device.
3-wire slave mode is active when NSSMD1 (SPI0CN.3) = 0 and NSSMD0 (SPI0CN.2) = 0. NSS is not
used in this mode, and is not mapped to an external port pin through the crossbar. Since there is no way of
uniquely addressing the device in 3-wire slave mode, SPI0 must be the only slave device present on the
bus. It is important to note that in 3-wire slave mode there is no external means of resetting the bit counter
that determines when a full byte has been received. The bit counter can only be reset by disabling and re-
enabling SPI0 with the SPIEN bit. Figure 25.3 shows a connection diagram between a slave device in 3-
wire slave mode and a master device.
Rev. 1.4
253
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