I have an Arduino Uno attached to a Parallax Boe-Bot shield, and plugged into the shield breadboard, the Parallax WX ESP8266 Wi-Fi SIP.
When I attach the DO/DI pins of the ESP8266 board to the RX/TX pins of the Uno, it does not work without a very narrow range of resisters in a voltage divider configuration. I'm trying to learn more about electronics and I would like to know, why these specific values in this specific case?
The DI/DO pins of the SIP are documented as "SIP - output high (set by Vin voltage, either 3.3 or 5.0 V), low 0 V. Input high >Vin/2, input low
When I attach the Parallax shield 5V to the SIP Vin, and the DO to a 220 resister to RX, and also DO to a 1K resister to ground, and DI to 220 to TX and also DI to 1K to ground, serial communications works fine.
If I use the shield 3.3V to SIP Vin, it does not work.
If instead of 220 / 1K, I use 470 / 2K, or 4.7K / 10K, for example, it doesn't work.
If I only set up the voltage divider on the RX pin, it doesn't work.
However, if instead, with either 5V or 3.3V to the SIP Vin, and no resisters, I can run the Uno TX direct to the SIP DI, and the SIP DO to a digital pin, and run that digital pin with SoftwareSerial, for example.
I can run both DI and DO directly with SoftwareSerial, and either 5V or 3.3V to supply Vin.
It's only when I want to connect DO/DI direct to use the Uno's UART (because I don't want serial-driving interrupts to interfere with servo pulses) that I have to carefully select these resisters (through trial and error) and be sure to power with the 5V. Some combinations get communication with errors, but just these specific values (out of the resisters I happen to have on hand) work.
I understand that it is something to do with 3.3V vs TTL serial, but I would like the details. (I will be picking up a logic level converter to try in place of the voltage divider.)
By "not working" -- in all but two cases, with fresh batteries, the SIP seems to get data from the Uno TX, and the DO lamp on the SIP flashes, but Serial.read() sees nothing on the Uno regardless of configured rates. In one of the two cases (I can't recall which) unintelligible, incomplete data is read on the RX pin, and in the last, good case -- where both pins have voltage dividers and the SIP is powered by the 5V line of the Arduino shield (for the Boe-bot) -- with those specific resister values -- the data from the SIP DO is received on the Serial, from pin RX.