f28335 question regarding SPI

Christian Ekman

Hi,

I'm working on implementing SPI to a project I'm working on. In certain cases I do not want the f28335 to keep sending data on the SIMO pin. Have I understood things correctly if I say that if I disable the TALK in the SPICTL register the SPISTE pin will be kept high all the time, and when I enable it again the SPISTE pin will start switching between low and high state again? If this is not correct, is there another way to do this?

//Christian

over 11 years ago

0 Christian Ekman over 11 years ago

Intellectual 340 points

I just read through the datasheet again and I interpret it like the SIMO pin will just have 0x0000 as output all the time if I disable TALK. Have I understood it correctly? 0x00 unfortunately is the slave's command word for write to register 00 and I don't want the slave to interpret any messages the wrong way...

//Christian

0 Bill Finger over 11 years ago in reply to Christian Ekman

Genius 3735 points

Christian,

There is more to the SPI bus than just the state of the SIMO pin. There is the clock, which must toggle to trigger the slave to sample the SIMO pin, and then there is the chip select, which should be low or the slave is not selected.

Disabling TALK should both stop the clock and raise the chip select.

Regards,

Bill

0 Christian Ekman over 11 years ago in reply to Bill Finger

Intellectual 340 points

Are you sure about that, Bill? Because it says in the SPI datasheet that disabling TALK still allows the device to receive data. Doesn't this mean that the clock needs to keep running and the chip select continues to alternate between low and high state?

Thanks again,

Christian

0 Christian Ekman over 11 years ago in reply to Christian Ekman

Intellectual 340 points

I just tried setting TALK = 0 and looked at it on the oscilloscope. Apparently what comes out of SIMO/MOSI is 0xFF which is the same as no operation on my slave meaning it will work most likely work perfectly. The clock and chip select keeps working as normal though so you can still receive data, just not send any.

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f28335 question regarding SPI