Lynxmotion Smart Servo(LSS) - Convert 3.3v to 5v Serial

[KurtE]

Currently I am playing around with a few different board designs to play with the newer Lynxmotion Smart Servos(LSS) with a hexapod.

I am playing around with a few different options for boards using T4.1, one that is setup to simply have a Arduino Shield, which you can plug in their
shield (http://www.lynxmotion.com/p-1156-lynxmotion-smart-servo-lss-adapter-board.aspx)
The only interesting thing about this shield is there are 4 IO pins of interest, 2 that drive the servos, and 2 that communicate with an XBee and these either take in or output 5V... Also Their XBee pins don't match up with one of our UARTS. So on that board I setup to build in the equivalent of the Sparkfun bidirectional level converter using 4 BSS138 and 8 resistors... And I have a solder jumper that allows me to redirect one IO pin to another to align XBee with a hardware Serial port....

But over the last day or so, I have been playing around with an all in one Kitchen Sink board Which 3d looks like:


In this case I could get rid of the level converters for XBee as they are 3.3v...
I still have the second pair on the board, going to IO pins 24/25 to maybe drive Neopixels or Dotstars and or experiment with FlySky IBus receiver.

But the question I have is: for the LSS servos which are full duplex, probably with max Baud of 1000000 am I better off leaving the communications using the BSS138 Bidirectional setup on the TX/RX pins,

Or should I convert these two pins like:
RX pin - simple 2 resistor divider: like 1K and 2K?

TX pin - Maybe something like the transistor used on LC to up convert for neopixel? like 74LV1T125 transistor like: https://www.digikey.com/en/products/detail/texas-instruments/SN74LV1T125DBVR/4555571
(Or other like what is on PropShield?)

Again just having some fun!

 

[mjs513]

Morning @KurtE
I usually use the https://www.adafruit.com/product/757, logic level converter:

This level shifter board combines the ease-of-use of the bi-directional TXB0108 with an I2C-compatible FET design following NXP's app note.

This breakout has 4 BSS138 FETs with 10K pullups. It works down to 1.8V on the low side, and up to 10V on the high side. The 10K's do make the interface a little more sluggish than using a TXB0108 or 74LVC245 so we suggest checking those out if you need high-speed transfer.

While we designed it for use with I2C, this works as well for TTL Serial, slow <2MHz SPI, and any other digital interface both uni-directional and bidirectional.

Guess it really depends on what you want to really use it for. The BSS138 I think is a low-voltage/low-current. Read for Dotstar they are talking about using the 74AHCT125 level converter. Design decisions never fun - well sometimes

 

[KurtE]

Thanks @mjs513,

Yes, that one is more or less the same as the SParkfun one. But the Adafruit one is bigger and different ordering of pins:

For the Shield version, I mentioned I actually did two versions, one that has all of the discrete components and one that you could plug in the Sparkfun board...
Which I posted a 3d preview on Robotshop forum... SInce then I went back to my T3.6 3d preview as the one I converted here is really big.. Will make a new one in the cad program soon. But may take me awhile to remember how it works...