Electronic – How do wire Active Low pins

circuit-designembeddedspi

I'm currently working with a small SPI Flash Memory chip attached to a microcontroller.

After having some problems interfacing with the memory, I took a closer look at the datasheet and realised that the RESET and WP (write-protect) pins are Active Low.

In terms of circuit design, am I right in thinking that if there is never a use for resetting or enabling write-protect, I should wire these to VCC (or in the case of a PCB, the power plane)? If so, is it better to have a resistor on these pins as well?

This is the pin description table

SYMBOL     DESCRIPTION
CS#        Chip Select
SI/SIO0    Serial Data Input (for 1 x I/O)
SO/SIO1    Serial Data Output (for 1 x I/O)
SCLK       Clock Input
WP#        Write Protection Active Low 
RESET#     Hardware Reset Pin Active low
HOLD#      To pause the device without deselecting the device or Serial Data Input & Output (for 4xI/O read mode)
VCC        Power Supply
GND        Ground

Best Answer

If the datasheet says these inputs can go all the way to Vdd (which most likely is the case), then yes, you can just connect them directly to Vdd if you don't want to use their features. There is no need for a resistor.

In early prototypes you might sometimes want to put a resistor there so that you can wire to the pin if you realize you want to control it anyway after making the first boards.

I've used SPI memories with pins like that a few times, and always just hard-wired them to the unasserted level (Vdd or ground).

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