Logic level clarification

Sensors NFC & RFID
1

Hi all,

I’ve recently purchased a SRTR and using it with DEV-15096 (CH340G). I’ve cut JP1 and using an external 5V 2A PSU to power it, i’ve found that its a bit unstable at 115200 baud rate, even 38400 is a bit unstable a well when using it at high power level. However it works pretty reliability at lower power levels.

On the CH340G to do i need to jumper the voltage to 5V rather than 3.3V for the logic levels? I’ve stumbled across this post, but haven’t been able to find a definitive answer on which voltage it needs to be.

https://learn.sparkfun.com/tutorials/si … de/discuss

Thanks for the pointer and info on the TXB0104. I understand now that the protection buffer performs level shifting on the RX/TX to the SRTR supply voltage. That is, if powered by 3.3V, the SRTR presents 3.3V logic; if powered by LiPo, 3.7V logic; if powered by 5V, 5V logic, etc…

Thanks!

2

The SRTR has on-board level converter (TXB0104). Internally it will work with 3V3 (generated by the M6Nano) which level is connected to one-side of the converter. The output logic level is set to the provided external power supply level (VCC). It does not matter whether that is coming through the serial connector / JP1 or connected to the separate pins. Normally that is around 5V.

On the CH340G the JP1 jumper is setting BOTH the VCC that goes external (not used in you case as you cut JP1 on the SRTR) as well as the VCC for the CH340G chip. Default it is set to 3V3, so you need to change that to 5V so the chip has the right level as well.

3

Awesome thanks, i will give it a crack.