MC9S12KG128 Datasheet, PDF (367/600 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 11 Serial Peripheral Interface (SPIV3)
The SPI Data Register is both the input and output register for SPI data. A write to this register allows a
data byte to be queued and transmitted. For a SPI configured as a master, a queued data byte is transmitted
immediately after the previous transmission has completed. The SPI Transmitter Empty Flag SPTEF in
the SPISR register indicates when the SPI Data Register is ready to accept new data.
Reading the data can occur anytime from after the SPIF is set to before the end of the next transfer. If the
SPIF is not serviced by the end of the successive transfers, those data bytes are lost and the data within the
SPIDR retains the first byte until SPIF is serviced.
11.4 Functional Description
The SPI module allows a duplex, synchronous, serial communication between the MCU and peripheral
devices. Software can poll the SPI status flags or SPI operation can be interrupt driven.
The SPI system is enabled by setting the SPI enable (SPE) bit in SPI Control Register 1. While SPE bit is
set, the four associated SPI port pins are dedicated to the SPI function as:
• Slave select (SS)
• Serial clock (SCK)
• Master out/slave in (MOSI)
• Master in/slave out (MISO)
The main element of the SPI system is the SPI Data Register. The 8-bit data register in the master and the
8-bit data register in the slave are linked by the MOSI and MISO pins to form a distributed 16-bit register.
When a data transfer operation is performed, this 16-bit register is serially shifted eight bit positions by the
S-clock from the master, so data is exchanged between the master and the slave. Data written to the master
SPI Data Register becomes the output data for the slave, and data read from the master SPI Data Register
after a transfer operation is the input data from the slave.
A read of SPISR with SPTEF = 1 followed by a write to SPIDR puts data into the transmit data register.
When a transfer is complete, received data is moved into the receive data register. Data may be read from
this double-buffered system any time before the next transfer has completed. This 8-bit data register acts
as the SPI receive data register for reads and as the SPI transmit data register for writes. A single SPI
register address is used for reading data from the read data buffer and for writing data to the transmit data
register.
The clock phase control bit (CPHA) and a clock polarity control bit (CPOL) in the SPI Control Register 1
(SPICR1) select one of four possible clock formats to be used by the SPI system. The CPOL bit simply
selects a non-inverted or inverted clock. The CPHA bit is used to accommodate two fundamentally
different protocols by sampling data on odd numbered SCK edges or on even numbered SCK edges (see
Section 11.4.3, “Transmission Formats”).
The SPI can be configured to operate as a master or as a slave. When the MSTR bit in SPI Control
Register1 is set, master mode is selected, when the MSTR bit is clear, slave mode is selected.
MC9S12KG128 Data Sheet, Rev. 1.15
Freescale Semiconductor
367