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Comments on Do we need to use pull up resistors in SPI lines when the ADC and Microcontroller are far apart

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Do we need to use pull up resistors in SPI lines when the ADC and Microcontroller are far apart

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I have an ADC board. This board will communicate with a STM32 develoment board via SPI interface. I am using jumper cables for connecting the ADC SPI lines to ST32 evaluation board.

Below is my ADC schematic. I gave the ADC schematic for review to the concerned analog devices team. The response received is given below.

We recommend adding the pull-up resistors on all lines. Generally, a 10k pull-up resistor can be used. However, if track/distance is long i.e. more than 5 to 8 inches then we would suggest using a 4.7k resistor. It could be useful to design these pull-up resistors in and they can be used as a ‘population option’.

Image_alt_text

For robustness and reliability (over time) Analog devices suggested to include pull-up resistors on the connector J4 going to the SPI pins.

May I know your thoughts.

ADC
posted 8 months ago
8mo ago
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newbie‭
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2 comment threads

So add pull ups? (4 comments)
How far? How fast? (2 comments)
So add pull ups?
Lundin‭ wrote 8 months ago · edited 8 months ago
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SPI idles high so that seems to be perfectly reasonable feedback, particularly the /CS pin. So why not just do that, what do you need our feedback for as well? A bunch of resistors add no cost especially since you already got a 10k reel in the BoM. As for 4k7 vs 10k that's pedantic, it won't matter in practice.

newbie‭ wrote 8 months ago
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I never see pull-up on all SPI lines. Some boards I have seen pull-up in CS lines. SPI is push-pull so I am confused why we need pull up resistors on SPI lines. I thought of asking experts. That is why I posted here.

Skipping 2 deleted comments.

Olin Lathrop‭ wrote 8 months ago
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Moderator reminder: Comments are not for answers. Such comments have been deleted.

Lundin‭ wrote 8 months ago
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newbie‭ If properly implemented the /CS (aka /SS) signal is the only one who must have a pull up, but in general it isn't a good idea to let digital signals "float" when not driven actively. In this case it seems that the signals go to a connector, which may perhaps get disconnected? Otherwise, even if they go straight to a MCU, the MCU might reset during which it doesn't drive the lines.