How to set up SPI MOSI only mode

Su Jin

Hi, everyone,

I plan to use an OLED display in my project, and SPI interface to do comunication between a MSP432P401R (the master) and the display (the slave.) An OLED is only able to receive commands and data from its master, but can not send any message to its master. Actually it does not have such a pin. However SPI is a two-direction communication, MOSI and MISO always occur simultaneously. So my question is which pin of OLED the master's MISO pin should connect with? Can I just simply ignore it and let master's MISO pin open?

Please help, and thank you very much.

S. Jin

over 10 years ago

0 Clemens Ladisch over 10 years ago

Guru 327080 points

A CMOS input pin should never be left open; see Implications of Slow or Floating CMOS Inputs.

However, that pin does not need to be an input; just don't connect it to the eUSCI module. (You can configure the pin as a GPIO output, or use it for something else.)

0 Su Jin over 10 years ago in reply to Clemens Ladisch

Intellectual 740 points

Thank you very much. But I do not quite understand

Clemens Ladisch said:
You can configure the pin as a GPIO output, or use it for something else

Do you mean I can use it for any thing I like but just not let it open?

S J

0 old_cow_yellow over 10 years ago in reply to Su Jin

Guru 58965 points

The User's Guide of various MSP430 all say the same thing about unused pins. The irony is, too many TI MSP430 "examples" and "sample code" do not practice their own advice. And the "user community" follows the bad examples religiously.

Is this situation the same for MSP432?

0 Su Jin over 10 years ago in reply to old_cow_yellow

Intellectual 740 points

Unfortunately the situation for MSP432 is too few code examples available, particularly in comparison with Arduino. In many cases we have to refer to Arduino or MSP430 code examples.

0 old_cow_yellow over 10 years ago in reply to Su Jin

Guru 58965 points

In that case, you probably can get your code to woke without worrying about the open input pins. They might draw a little more current or start to oscillate.

In the early days of CMOS, an open input pin might latch-up.

0 Clemens Ladisch over 10 years ago in reply to Su Jin

Guru 327080 points

To answer your actual question: you don't need to connect the master's MISO signal to anything.

Configuring the actual pin as an output (or enabling a pull-up/-down resistor, or connecting the pin to VCC/GND) has nothing to do with SPI itself, it just affects the power consumption.

0 Su Jin over 10 years ago in reply to old_cow_yellow

Intellectual 740 points

Thank you very much. I really do not know which of leaving it open or connecting it to GND is OK, and I'll try each of them.
S. Jin

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How to set up SPI MOSI only mode