Logic Level Converter, Arduino and Cellphone

Question

I am trying to interface a 6150 Nokia cellphone with an Arduino Duemilanove board, as some have already done through the FBUS protocol (see here for example).

The FBUS protocol uses 2.8V logic, so I bought Sparkfun's logic level converter to interface with the 5V from the Arduino, right ?

They say:

The level converter is very easy to use. The board needs to be powered from the two voltages sources (high voltage and low voltage) that your system is using. High voltage (5V for example) to the 'HV' pin, low voltage (2.8V for example) to 'LV', and ground from the system to the 'GND' pin.

Thus

HV  <->  Arduino 5V
GND <->  Arduino GND

LV  <->  ??
GND <->  ??

TXI <->  Phone TX    (Low Voltage)
RXO <->  Phone RX    (Low Voltage)
TXO <->  Arduino RX  (High Voltage)
RXI <->  Arduino TX  (High Voltage)

The problem I have is with the low voltage source: there is no power output on the phone.

Can I power the converter LV/GND from another 2.8V source ? Would it be ok with the Arduino 3.3V source ?

Answer 1

That website has a photo which shows "2-GND". This is the ground that you want to connect to the ground that you have for everything else.

As far as the 2.8v source, you can use any source you want. The voltage is just used to reference the serial signal against. If you tie it to the 3.3V it will probably work. I am not sure of the specifics of that voltage shifter, but I assume it will treat 2.8v on the signal line as being a logic high when comparing to 3.3v.

Answer 2

You could use a resistive voltage divider to create the 2.8 Volt source from the 5 Volt supply (HV on the level shifter board) available from the Arduino.

(image src: Play-Hookey)

Or you could use a linear voltage regulator such as the LP3987-2.8 (SMT) or similar.