C8051F960-B-GM Datasheet, PDF (426/492 Pages) Silicon Laboratories – Ultra Low Power 128K, LCD MCU Family

C8051F96x
31.1. Signal Descriptions
The four signals used by SPI1 (MOSI, MISO, SCK, NSS) are described below.
31.1.1. Master Out, Slave In (MOSI)
The master-out, slave-in (MOSI) signal is an output from a master device and an input to slave devices. It
is used to serially transfer data from the master to the slave. This signal is an output when SPI1 is
operating as a master and an input when SPI1 is operating as a slave. Data is transferred most-significant
bit first. When configured as a master, MOSI is driven by the MSB of the shift register in both 3- and 4-wire
mode.
31.1.2. Master In, Slave Out (MISO)
The master-in, slave-out (MISO) signal is an output from a slave device and an input to the master device.
It is used to serially transfer data from the slave to the master. This signal is an input when SPI1 is
operating as a master and an output when SPI1 is operating as a slave. Data is transferred most-
significant bit first. The MISO pin is placed in a high-impedance state when the SPI module is disabled and
when the SPI operates in 4-wire mode as a slave that is not selected. When acting as a slave in 3-wire
mode, MISO is always driven by the MSB of the shift register.
31.1.3. Serial Clock (SCK)
The serial clock (SCK) signal is an output from the master device and an input to slave devices. It is used
to synchronize the transfer of data between the master and slave on the MOSI and MISO lines. SPI1
generates this signal when operating as a master. The SCK signal is ignored by a SPI slave when the
slave is not selected (NSS = 1) in 4-wire slave mode.
31.1.4. Slave Select (NSS)
The function of the slave-select (NSS) signal is dependent on the setting of the NSSMD1 and NSSMD0
bits in the SPI1CN register. There are three possible modes that can be selected with these bits:
1. NSSMD[1:0] = 00: 3-Wire Master or 3-Wire Slave Mode: SPI1 operates in 3-wire mode, and NSS is
disabled. When operating as a slave device, SPI1 is always selected in 3-wire mode. Since no select
signal is present, SPI1 must be the only slave on the bus in 3-wire mode. This is intended for point-to-
point communication between a master and one slave.
2. NSSMD[1:0] = 01: 4-Wire Slave or Multi-Master Mode: SPI1 operates in 4-wire mode, and NSS is
enabled as an input. When operating as a slave, NSS selects the SPI1 device. When operating as a
master, a 1-to-0 transition of the NSS signal disables the master function of SPI1 so that multiple
master devices can be used on the same SPI bus.
3. NSSMD[1:0] = 1x: 4-Wire Master Mode: SPI1 operates in 4-wire mode, and NSS is enabled as an
output. The setting of NSSMD0 determines what logic level the NSS pin will output. This configuration
should only be used when operating SPI1 as a master device.
See Figure 31.2, Figure 31.3, and Figure 31.4 for typical connection diagrams of the various operational
modes. Note that the setting of NSSMD bits affects the pinout of the device. When in 3-wire master or
3-wire slave mode, the NSS pin will not be mapped by the crossbar. In all other modes, the NSS signal will
be mapped to a pin on the device. See Section “27. Port Input/Output” on page 351 for general purpose
port I/O and crossbar information.
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