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I'm designing a small development board (from schematic design to PCB), centered around an ATmega32U4 MCU, and a Quectel M95 GSM module. I'm using the M95 Hardware Design Manual (http://www.quectel.com/UploadImage/Downlad/M95_Hardware_Design_V1.3.pdf) and there's a part that I don't understand for the UART reference design, to connect the M95 to the ATmega32U4 MCU.

It firstly shows this simple connection: UART

Then it's got this one: enter image description here

And the DC characteristics for the UART port on the M95: enter image description here Where VDD_EXT is 2.8V.

I don't particularly understand which reference design to go with? The first one makes sense, you just connect the UART pins from the MCU to the GSM module. The second one also makes sense: if your MCU is based on 3.3V (which mine is), then this is the reference design to follow in order to have a voltage drop of 2.8V, in order to satisfy the DC characteristics listed above (?). I'm unsure which one to go with? Does it matter?

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    \$\begingroup\$ If you're laying out a PCB, lay out the second one. You can always build the first by using 0 ohm for 1K and omitting the 5.6k resistors. \$\endgroup\$ – Brian Drummond Nov 5 '16 at 11:06
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The first diagram basically says that you have to connect RXD to TXD in order to have a full-duplex communication. It's just a connectivity diagram.

The second one includes a voltage divider for voltage translation. Let say it's an application circuit.

Both diagrams are coherent.

Just follow the second one and set the voltage divider accordingly with your ICs

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    \$\begingroup\$ That is correct, the purpose of the first image is to show which pins should be connected. Figure 19 shows a minimal solution while Figure 20 with hardware flow control. The second is a level shifting schematic, which is reference design in a certain case (3.3 V and 3 V). Figure 24 shows the 5 V level shifting and Figure the RS232 level shifting. When making your own schematic these should be considered as examples. \$\endgroup\$ – Bence Kaulics Nov 5 '16 at 14:15

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