NXP Semiconductors freescale semiconductor MC13211, Reference Manual

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MC1321x Serial Peripheral Interface (SPI)
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 4-7
4.6.2 MCU SPI Baud Rate Generation
As shown in Figure 4-5 , the clock source for the SPI baud rate generator is the bus clock. The three
prescale bits (SPPR2:SPPR1:SPPR0) choose a prescale divisor of 1, 2, 3, 4, 5, 6, 7, or 8. The three rate
select bits (SPR2:SPR1:SPR0) divide the output of the prescaler stage by 2, 4, 8, 16, 32, 64, 128, or 256
to get the internal SPI Master Mode bit-rate clock.
Figure 4-5. MCU SPI Baud Rate Generation
4.7 MCU SPI Functional Description
A SPI transfer is based on an 8-bit operation. A SPI transfer is initiated by checking for the SPI transmit
buffer empty flag (SPTEF = 1) and then writing a byte of data to the SPI data register (SPI1D) in the master
SPI device. When the SPI shift register is available, this byte of data is moved from the transmit data buffer
to the shifter, SPTEF is set to indicate there is room in the buffer to queue another transmit character if
desired, and the SPI serial transfer starts.
During the SPI transfer, data is sampled (read) on the MISO1 pin at one SPSCK edge and shifted, changing
the bit value on the MOSI1 pin, one-half SPSCK cycle later. After eight SPSCK cycles, the data that was
in the shift register of the master has been shifted out the MOSI1 pin to the slave while eight bits of data
were shifted in the MISO1 pin into the master’s shift register. At the end of this transfer, the received data
byte is moved from the shifter into the receive data buffer and SPRF is set to indicate the data can be read
by reading SPI1D. If another byte of data is waiting in the transmit buffer at the end of a transfer, it is
moved into the shifter, SPTEF is set, and a new transfer is started.
Normally, SPI data is transferred most significant bit (MSB) first. If the least significant bit first enable
(LSBFE) bit is set, SPI data is shifted LSB first. For this embedded SPI interface, the LSB First Mode must
not be used.
The MCU SPI module can be configured as a slave, however, this mode is not allowed in the MC1321x.
Because the transmitter and receiver are double buffered, a second byte, in addition to the byte currently
being shifted out, can be queued into the transmit data buffer, and a previously received character can be
in the receive data buffer while a new character is being shifted in. The SPTEF flag indicates when the
transmit buffer has room for a new character. The SPRF flag indicates when a received character is
available in the receive data buffer. The received character must be read out of the receive buffer (read
SPI1D) before the next transfer is finished or a receive overrun error results.
In the case of a receive overrun, the new data is lost because the receive buffer still held the previous
character and was not ready to accept the new data. There is no indication for such an overrun condition
so the application system designer must ensure that previous data has been read from the receive buffer
before a new transfer is initiated.
DIVIDE BY
2, 4, 8, 16, 32, 64, 128, or 256
DIVIDE BY
1, 2, 3, 4, 5, 6, 7, or 8
PRESCALER CLOCK RATE DIVIDER
SPPR2:SPPR1:SPPR0 SPR2:SPR1:SPR0
BUS CLOCK
MASTER
SPI
BIT RATE