ATSAMD & SD SPI or SPI FLASH

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Hello.

In the datasheet of ATSAMD20J18 it states the following:

 

7.3.2 I/O Pins
After power-up, the I/O pins are tri-stated.

 

 

As far as I know, a tri-stated pin means that the pin on power-up of the processor goes to high-impedance state (not low or high). On the processor in my designs, I sometimes connect either an SD card interface or an SPI flash. Does that mean that I should connect the CS (chip select) pins of these to pull-up resistors? If not, is it possible that I might clock random data from the pin into the SD of SPI flash chip?

 

So far, I was never connecting pull-ups on CS and it didn't seem to had any problem. After power-up I was first enabling the internal pullup. Maybe I face a problem in the future and I will not know of it?

 

Thank you very much.

 

 

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it is normally good practice to put an external pullup on the select pin(s). If you have multiple devices on the one spi bus, then you want to ensure multiple devices are not accidently enabled in the startup phase. As well, some devices may consume more power when enabled, so in a low power design, this might be a problem.

 

Similarly, things like mosfets etc need pullups/pulldowns to ensure they don't accidently activate in the startup phase.

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So, if I understood correctly, in a low power design they usually try to avoid putting external pullup resistors if possible?

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I don’t think we can generalise as there is many variables.