Arduino Serial TX to iPhone RX voltage drop

arduinolevel-shiftingserial

Articles I read about this tell me to hook up a 1KOhm resistor between the Arduino's TX and the iPhone's RX. I understand this is because Arduino's signal is 5V and I want 3.3V going to the iPhone.

My question is, where did they arrive at this value of 1KOhm? Is it documented somewhere that the serial signals are 1.7mA?

Best Answer

someone probably mucked with resistors until the voltage came right, or maybe there's a pullup/down resistor sitting there on the input.

Or it's a high impedance input and 1.7mA is the most they feel comfortable pushing through the input's protection diodes

It depends a lot on what the input circuit and impedance is - you might be better off (and safer) building a voltage divider (2 resistors of your own) you really don't want to be pulling inputs above their Vdd - you end up dumping that current into the protection diodes and making stuff hot (or mildy warm) - better off to start with a simple voltage divider and then looking at the signal levels on a 'scope - tweak to get the result near to 3.3v but lower