C8051F310 datasheet(171/224 Pages) ETC1

171 / 224 page

Rev. 1.6

171

C8051F310/1/2/3/4/5/6/7

16.2. 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-mas-

ter mode, slave devices can be addressed individually (if needed) using general-purpose I/O pins.

Figure 16.2 shows a connection diagram between two master devices 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 16.3

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 16.4 shows a connection diagram for a master device in

4-wire master mode and two slave devices.

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