C8051F970-A-GM Datasheet, PDF (330/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
28.2. SPI0 Master Mode Operation
A SPI master device initiates all data transfers on a SPI bus. SPI0 is placed in master mode by setting the Master
Enable flag (MSTEN, SPI0CN.6). Writing a byte of data to the SPI0 data register (SPI0DAT) when in master mode
writes to the transmit buffer. If the SPI shift register is empty, the byte in the transmit buffer is moved to the shift
register, and a data transfer begins. The SPI0 master immediately shifts out the data serially on the MOSI line while
providing the serial clock on SCK. The SPIF (SPI0CN.7) flag is set to logic 1 at the end of the transfer. If interrupts
are enabled, an interrupt request is generated when the SPIF flag is set. While the SPI0 master transfers data to a
slave on the MOSI line, the addressed SPI slave device simultaneously transfers the contents of its shift register to
the SPI master on the MISO line in a full-duplex operation. Therefore, the SPIF flag serves as both a transmit-
complete and receive-data-ready flag. The data byte received from the slave is transferred MSB-first into the
master's shift register. When a byte is fully shifted into the register, it is moved to the receive buffer where it can be
read by the processor by reading SPI0DAT.
When configured as a master, SPI0 can operate in one of three different modes: multi-master mode, 3-wire single-
master mode, and 4-wire single-master mode. The default, multi-master mode is active when NSSMD1
(SPI0CN.3) = 0 and NSSMD0 (SPI0CN.2) = 1. In this mode, NSS is an input to the device, and is used to disable
the master SPI0 when another master is accessing the bus. When NSS is pulled low in this mode, MSTEN
(SPI0CN.6) and SPIEN (SPI0CN.0) are set to 0 to disable the SPI master device, and a Mode Fault is generated
(MODF, SPI0CN.5 = 1). Mode Fault will generate an interrupt if enabled. SPI0 must be manually re-enabled in
software under these circumstances. In multi-master systems, devices will typically default to being slave devices
while they are not acting as the system master device. In multi-master mode, slave devices can be addressed
individually (if needed) using general-purpose I/O pins. Figure 28.2 shows a connection diagram between two
master devices and a single slave in multiple-master mode.
3-wire single-master mode is active when NSSMD1 (SPI0CN.3) = 0 and NSSMD0 (SPI0CN.2) = 0. In this mode,
NSS is not used, and is not mapped to an external port pin through the crossbar. Any slave devices that must be
addressed in this mode should be selected using general-purpose I/O pins. Figure 28.3 shows a connection
diagram between a master device in 3-wire master mode and a slave device.
4-wire single-master mode is active when NSSMD1 (SPI0CN.3) = 1. In this mode, NSS is configured as an output
pin, and can be used as a slave-select signal for a single SPI device. In this mode, the output value of NSS is
controlled (in software) with the bit NSSMD0 (SPI0CN.2). Additional slave devices can be addressed using
general-purpose I/O pins. Figure 28.4 shows a connection diagram for a master device and a slave device in 4-
wire mode.
Rev 1.0
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