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SPI question

Zhuoyi Tang
Prodigy 120 points

I wanted to have ez430-RF2500 chip and CC2544 chip communicating by SPI. I went through slau144i document for ez430-RF2500.

1. In chapter 16, page 451, the master mode of SPI has a UCxSTE connected to SS. Is UCxSTE the same as slave select? Then why is slave select an input in master? Can someone briefly explain how does slave select work in SPI communication?

2. When does SS work after writing data to UCxTXBUF?

Thanks

  • 0
    Expert 1490 points

    Zhuoyi,

    This may help with your understanding of SPI and STE.  

    The STE pin is basically used as input both on SPI master and SPI slave mode: http://processors.wiki.ti.com/index.php/MSP430_FAQ#How_does_the_4_pin_SPI_work_on_USCI_module.3F

    If you want to use the USCI module in SPI mode to communicate with external flash memory, which is basically it should act as SPI master, the best way is to use 3 pin mode and a GPIO for controlling the slave enable pin of the external flash memory.

    Take care,

    Jon

  • 0
    Guru 227245 points

    Zhuoyi Tang said:
    master mode of SPI has a UCxSTE connected to SS. Is UCxSTE the same as slave select?

    No. I don't know the code, but often enough, the STE pin is not used fo rSTE funcitonality but rather used as simple GPIO pin (independently of the USCI module) to controle the SS 9r CS pin of the slave. Just because it is located next to the SOMI/SIMP/SCL pins.

    If you are single SPI master, use 3-wire mode. 4-wire mode is for slave operation or if you are secondary master in a multi-master system.

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