1.8V UART with Voltage divider
Connect 3.3/5V UART hardware to a 1.8V device using a voltage divider
We recommend buying suitable 1.8V equipment
3.3V: Z1 = 20k; Z2 = 20k 5.0V: z1 = 20k; Z2 = 47k
These values should be safe even if the resistors vary by 5%
Test the output Voltage before you send data to 1.8V ports
Connection of Z2 to GND is very important. If it has a bad connection Vout can get close to Vin
You need to have a common GND. The easiest method is connecting UART-GND and SRT-GND.
GND connection isn't shown in the picture.
Raspberry Pi's use 3.3V UART. Surface RT only uses 1.8V.
Raspberry Pi can receive 1.8V signals without a problem. But the Surface doesn't want to receive a 3.3V Signal. Therefore we need a levelshifter. A simple and cheap solution is to use a voltage divider, as shown above.
connection example with Voltage divider
picture of a breadboard setup