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I have Quectel EC25 module comes with GPRS and GNSS in built, this module needs supply of 3.3V and it's UART works at 1.8V (default Baudrate is 115200) - But i have a processor ATMEGA2560V running at 8Mhz on 3.3V supply. I am trying to interface that module with my processor, I have used voltage divider circuit with 1M Ohms resistors to pull down 3.3V of processor to 1.8V, I can see some random values comes in the Serial Monitor and i am unable to send any AT commands.

How do i solve this.? is this need any more reliable Resistor divider circuit.? and i don't have source to get 1.8V to 3.3V level shifter :(

Please help

Thank you

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  • \$\begingroup\$ Draw an NPN transistor. Rotate it 90 degress clockwise (i.e. emitter to the left, collector to the right, base to the up). Tie emitter with 1k to the 1.8V. Tie base with 1k to the 1.8V. Tie collector with 1k to the 3.3V. This circuit is called single transistor bidirectional buffer. Emitter goes to GPRS module's TX and collector goes to ATmega's RX. If GPRS module's pins are 3V tolerant (and I think they are) then you can connect ATmega's TX to module's RX directly. Otherwise, you need one more translator with similar connection. \$\endgroup\$ Commented Oct 7, 2017 at 18:15

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  1. Do not use meg ohm resistors for the voltage divider from the 3.3V to 1.8V UART direction flow. Something with resistors more in the 3.3K to 10K range would be much more appropriate.

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  1. For the 1.8V to 3.3V operational direction you can easily make a level translator with a couple of transistors and resistors. 2N3904 NPN transistors will work great.

enter image description here

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  • \$\begingroup\$ Question: to make your explanation super-clear when you say "TO_XXX_UART" you mean "UART_RX in the device at XXX Volt" and when you say "FROM_XXX_UART" you mean "UART_TX in the device at XXX Volt", correct? \$\endgroup\$
    – Davide
    Commented Mar 6, 2023 at 0:24
  • \$\begingroup\$ @Davide - These circuit ideas relate to signal direction and need for translation level. There is no direct inference to anything related to any particular signals of the TTL UART signal lines. You need to comprehend that depending upon application that RX of one device may connect to TX of another device. Likewise an RTS from once device may be connected to CTS of the other device. Understand the connection flows and the above circuits will cover all the interconnections. - - - - I fixed the transistor typo in the text. \$\endgroup\$ Commented Mar 7, 2023 at 4:36
  • \$\begingroup\$ I deleted the typo comment, no need for new readers to know all history of the answer. Re: RX and TX in UART, yes, I know how they connect, and this circuit must sit in between. I am fairly certain on how to place this circuit in between, but since there is no "FROM" or "TO" in UART, I wanted to make it super-clear. Thanks! \$\endgroup\$
    – Davide
    Commented Mar 8, 2023 at 13:17
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You can also choose an IC to do this under IC>Logic>translaters,Level Shifters and find the right one out of a couple thousand types. Possible non-inverting Push-pull. https://www.digikey.ca/products/en/integrated-circuits-ics/logic-translators-level-shifters/732?k=level+shifter&k=&pkeyword=level+shifter&pv2094=i1&pv1912=3&FV=ffe002dc%2C1ddc0002&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

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I suggest you to follow EC25 hardware design guide. you can either use a translator ic(like TXS0108EPWR) or you can use a Transistor Circuit. Both options are clearly stated in EC25 Hardware Design Guide pg.44-45

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