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while going through one schematics , I found that we are using Pull up resistor for MISO, MOSI, CS in SPI connection between IC and Flash? Why we are using pull up for SPI connections? Is it any related to FLASH Programming?

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  • \$\begingroup\$ It may define the logic levels when (whatever IC is) hasn't, to help prevent accidental data corruption. \$\endgroup\$
    – user16324
    Jul 1, 2021 at 11:29
  • \$\begingroup\$ You may find out that similar pull-ups/pull-downs are missing for other CMOS input loads - would be wise to add them perhaps :) Those resistors are absolutely necessary unless the MCU has built-in weak pull-ups, or the loads have them (loads = input pins of various CMOS chips, or mosfets in general). Floating CMOS inputs are bad news all around. \$\endgroup\$ Jul 1, 2021 at 19:25

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The main thing is, floating CMOS inputs are bad, and they float when nothing drives them, so it is best to pull them up or down.

You are free to pull the lines up or down (with one exception), but it might depend on a lot of things which you don't mention, like the MCU it is connected to.

The only thing that is very important is that the CS pin should be pulled up to keep the flash chip deselected during powerup, powerdown, and in general when the MCU is being reset or reprogrammed, so that the SPI flash never accidentally sees commands like write or erase.

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