I have just plugged one of those 4-channel 5v<->3v level shifters to Pico inputs.

I have 5v connected to the 5v side and 3.3v connected to the other.

With the 5v-side pins open-circuit, my Pico inputs are high. Am I supposed to use pulldowns on the 5v side?

TTL will always assume high for a disconnected input. That's how it's designed to work. As you are (effectively) linking TTL to a 3V3 input then what you are seeing is correct. You can use pull-down resistors if you wish or you can use active-low inputs.

Phenix,

These level shifter boards are BI-directional, and intended for open drain/open collector busses like I2C bus, SM bus, or even PS2 keyboard/mouse interfaces.

Adding a pull-down will disable the bi-directional function.

Since you are playing with quadrature encoders, I assume you are thinking of using these to connect an encoder to a picomite. Although this may work, the pullup on those board may limit their use (for a quadrature encoder, where rising and falling edge need to be timing accurate) to 50kHz.
Note they can support 400kHz I2C bus, but for I2C bus the falling edge is dominant, the rising edge timing is less critical. For quadrature 4/4 the rising edge is also important.

If my assumption is true, you need a uni-directional 5V->3.3V translator. And in many cases a simple voltage divider (i.e. 1k/2.2k) will do the job fine, and will work up to 1MHz or more.

Volhout

  Volhout said  

If my assumption is true, you need a uni-directional 5V->3.3V translator. And in many cases a simple voltage divider (i.e. 1k/2.2k) will do the job fine, and will work up to 1MHz or more.
Volhout

Volhout,

Your psychic powers are starting to scare me  

Naturally, I'm not in the same location as my proto-boards, components, etc. and so I just got back to my desk with what I need.
Yeah, going for the voltage divider.

I confess that; after reading other forums where 5V signals haven't damaged the Pico, I wanted to be safe, even though I have something like 15 Picos lying around and can afford to lose one. Very tempting when you just wanna test something  

I built a resistor logic level dropper for arduino to spi ili9341, one way, just display. worked fine but replaced with bi-directional 8 channel for touch.

Oh wait, I have a bunch of 74HC4050s somewhere...This is uni-directional.