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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

Theory of Voltage divider

Simple voltage divider

Calculating a voltage divider

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.

Silicon Labs CP2102

Raspberry Pi

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.

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

Theory of Voltage divider

Simple voltage divider

Use a to get TX 3.3V / 5V down to 1.8V

Vout = \frac{Z2}{Z1+Z2} * Vin

Calculating a voltage divider

3.3V: Vout = \frac{20k\Omega}{20k\Omega + 20k\Omega} * 3.3V = 1.65V

5.0V: Vout = \frac{20k\Omega}{20k\Omega + 47k\Omega} * 5.0V \approx1.5V

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.

Silicon Labs CP2102

Raspberry Pi

See for further on information on how to setup your RPI.

Raspberry Pi's use 3.3V UART. Surface RT only uses 1.8V.