UART logic level

[net_worker]

I have a gps module which uses uart for connection to the mcu and I wany to use it with the teensy 4.1. The problem here is that the uart of the gps is 1.8v and I am not sure if the teensy will be able to read the data from the gps?

 

[MarkT]

Are you needing duplex serial or only simplex?

There are some chips that can do 1.8V->3.3V level translation, but maybe someone does a module that can level shift in both directions 1.8V <-> 3.3V these days? Perhaps some of the 3.3V <-> 5V also work at lower voltages?

Its worth checking if the GPS module is 3.3V tolerant if you also sending data to it as then a level-shifter in that direction isn't needed.

BTW you have cross-posted, which means the details of the GPS are in a different thread to this one--this is not helpful, please abandon the other thread and post all details here... Cross posting usually leads to confusion and is counter-productive for everyone.

 

[net_worker]

I need a duplex communication as the moduel also has GSM and I need to send AT commands to it, I will attach the datasheet again. Also is it possible to delete the older thread?

 

[defragster]

yes (re p#2) - check - UART spec puts both ends of Rx and Tx at high Voltage on Stop - so the 1.8V GPS will need to tolerate 3.3V from Teensy.

Thread delete is rare - just mark with link to this thread as Active - it may get closed ...

 

[net_worker]

I am attaching the datasheet of the MCU used in the Teensy 4.1, I am not sure about 4.3.2.1 General purpose I/O AC parameters in the datasheet. (Page 39 & 40) - the file is too big so here is link to digikey https://www.nxp.com/docs/en/nxp/data-sheets/IMXRT1060CEC.pdf

 

[BriComp]

I see that the Quectel device provides 1.8V pins to be used for pull ups for OpenCollector devices.
You need either a level shifter to 1.8V or a buffer with an O/C output.

 

[net_worker]

I found these two: https://www.digikey.bg/en/products/detail/nexperia-usa-inc/74LVC2T45GT-115/2530598 https://store.comet.bg/Catalogue/Product/46370/ - both are bi-directional and are 1.8 to 3.3V

 

[PaulStoffregen]

I need a duplex communication as the moduel also has GSM and I need to send AT commands to it, I will attach the datasheet again.

I looked briefly at the datasheet and found this diagram on page 29 (30th page of the PDF). Looks like all the signals are unidirectional. I'd recommend looking for unidirectional buffers from 1.8V to 3.3V, and from 3.3V to 1.8V.

 

[KenHahn]

The 74LVC2T45 parts will work fine. You can just use 3 of the chips since each has 2 channels and permanently tie the directional control pins to high or low to get the direction of flow you need.

I use these parts to make a high speed SPI bus level translator that works in the same fashion. https://protosupplies.com/product/hi-speed-spi-logic-level-converter-module/

 

[net_worker]

Solved it !

 

[jmarsh]

You're much better off using either a unidirectional shifter or one with directional control. TXS0108 is known to have problems detecting the direction of a signal when in push-pull mode.

(Also why are there both pull-up and pull-down resistors attached to OE?)

 

[PaulStoffregen]

If you do use the TXS part, keep the wires short and route the GND return wires close or use a 4 layer board with GND plane.

We've previously heard people report problems with those TXS level shifters when using longer wires.

 

[net_worker]

I choosed it primarily because it was recommended in the datasheet of the GPS. The pcb will be 6/8 layers (at least 2 of them will be ground planes).