Translated signals (multiple level) with LS7366R

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Translated signals (multiple level) with LS7366R

Post by Markkim » Fri Sep 10, 2021 2:20 am

Hello all,
I’m using 2 LS7366R ICs for interfacing with our two encoders. Now, our encoders operate at 5V and we are supplying 5V VCC to the LS7366 chips as well. However, our control unit consists of Rpi running @3.3V. So, I wanted to know is it okay if I just use the level translators and connect it to Pi?
Have a look at the schematic below.The idea is that both counter ICs U9 and U15 will be connected to the same MISO, MOSI and SCK signals ( which are thereafter connected to level translators.The slave select signals are different though.
( LS7366R Technical Specifications Source: ... cuits.html )

Can anyone confirm that the MOSI/MISO lines will not behave in any unexpected way or is there a better way of achieving such a functionality in our design? Thanks in advance.
LS7366 - Level Translators.jpg
Encoder - LS7366 Connections.jpg

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Re: Translated signals (multiple level) with LS7366R

Post by haklesup » Thu Sep 16, 2021 5:12 pm

I haven't dug into the meat of your specific application but the crux of level shifting comes from analysis of the DC specifications part of the datasheet. In particular, you need to verify that the output logic levels from the first stage are compatible with the input logic levels of the second stage. you need to obtain datasheets of all devices and look at the VIL, VIH, VOL and VOH specs and switching threshold.

Its often easier to shift up since 3V outputs will work on 5V CMOS inputs for the most part but shifting down can be tricky, the 3V part needs input protection that can tolerate more than VDD and most CMOS parts don't do that (they will shunt the signal starting at VDD+0.5V or sot). to shift up LS (5V bipolar) logic to 3.3V CMOS logic often works,

your best bet is to select a level shifting IC which is often just a buffer or pullup transistor with inputs and outputs having extended logic levels and adjusted switching threshold

Do an image search for "Logic Shifting Levels" and look for the color coded charts that look a little like bar charts, they help to visualize the logic compatibilities between various semiconductor families

This tutorial looks helpful to this topic, the same artical has some low cost level shifter modules you can look into how they work.

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