I'm trying to connect ZTE MG2639 to 5V AVR (or USB<->RS232). This module is using 2.8V at IO. How can I do that? Normal voltage divider?
And how to check if communication is OK in Putty? Just type "AT"? Or "AT" and press something? It's my first time playing with AT Commands.
For the 2.8V to 5V, since 2.8V is below most 5V system's \$V_{IH}\$, which is most often \$0.6V_{CC}\$, or 3V, you will need a buffer of some form.
The simplest method is to use any logic gate from the 74HCT family (they have TTL input thresholds) which results in a non-inverted signal. Either a specific non-inverting buffer, or any gate whose inputs can be arranged to mirror their value to the output, such as an AND gate (74HCT08) with both inputs tied together.
For the 5V -> 2.8V a simple voltage divider would do the trick.
As for testing, the best command to send would be "ATI" and pressing return should result in information about the module being returned.
The full AT command set can be seen here: http://download.maritex.com.pl/pdfs/wi/MG2639AT.pdf
You may connect them over a non-inverting Buffer to change the levels from 2.8V to (I guess) 5V (74hct34 or similar). The other way a voltage divider is sufficient. Or if you need a cleaner solution, have a look at level translators (for example the ones of TI)
