M68HC11E_SPI - Chapter 8 Serial Peripheral Interface(SPI...

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M68HC11E Family Data Sheet, Rev. 5.1 Freescale Semiconductor 119 Chapter 8 Serial Peripheral Interface (SPI) 8.1 Introduction The serial peripheral interface (SPI), an independent serial communications subsystem, allows the MCU to communicate synchronously with peripheral devices, such as: Frequency synthesizers Liquid crystal display (LCD) drivers Analog-to-digital (A/D) converter subsystems Other microprocessors The SPI is also capable of inter-processor communication in a multiple master system. The SPI system can be configured as either a master or a slave device. When configured as a master, data transfer rates can be as high as one-half the E-clock rate (1.5 Mbits per second for a 3-MHz bus frequency). When configured as a slave, data transfers can be as fast as the E-clock rate (3 Mbits per second for a 3-MHz bus frequency). 8.2 Functional Description The central element in the SPI system is the block containing the shift register and the read data buffer. The system is single buffered in the transmit direction and double buffered in the receive direction. This means that new data for transmission cannot be written to the shifter until the previous transfer is complete; however, received data is transferred into a parallel read data buffer so the shifter is free to accept a second serial character. As long as the first character is read out of the read data buffer before the next serial character is ready to be transferred, no overrun condition occurs. A single MCU register address is used for reading data from the read data buffer and for writing data to the shifter. The SPI status block represents the SPI status functions (transfer complete, write collision, and mode fault) performed by the serial peripheral status register (SPSR). The SPI control block represents those functions that control the SPI system through the serial peripheral control register (SPCR). Refer to Figure 8-1 , which shows the SPI block diagram. 8.3 SPI Transfer Formats During an SPI transfer, data is simultaneously transmitted and received. A serial clock line synchronizes shifting and sampling of the information on the two serial data lines. A slave select line allows individual selection of a slave SPI device; slave devices that are not selected do not interfere with SPI bus activities. On a master SPI device, the select line can optionally be used to indicate a multiple master bus contention. Refer to Figure 8-2 .
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Serial Peripheral Interface (SPI) M68HC11E Family Data Sheet, Rev. 5.1 120 Freescale Semiconductor Figure 8-1. SPI Block Diagram 8.4 Clock Phase and Polarity Controls Software can select one of four combinations of serial clock phase and polarity using two bits in the SPI control register (SPCR). The clock polarity is specified by the CPOL control bit, which selects an active high or active low clock, and has no significant effect on the transfer format. The clock phase (CPHA) control bit selects one of two different transfer formats. The clock phase and polarity should be identical for the master SPI device and the communicating slave device. In some cases, the phase and polarity
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M68HC11E_SPI - Chapter 8 Serial Peripheral Interface(SPI...

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