FrSky D6FR specifications are wrong!

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FrSky D6FR specifications are wrong!

Postby soopy » Wed Mar 23, 2011 10:05 am

Hi all,

I'm re-posting here what I posted on RCGroups. I have found that FrSky have tried to work around some differences between the D8R and D6FR hardware to try and make the D6FR compatible with the tools already designed for the D8R V2. I believe they have made an error in doing so, see below:

I did some more measurements today and can now confirm the following for the D6FR:

Input AD2 is not a true 1:1 input as stated in the manual. It has an internal 1:6 voltage divider made up of a 7k5 and 1k5 resistor. A full scale reading of 255 corresponds to an input voltage of 3.3V at the AD2 input pin. This results in an actual voltage of 0.55V on the processor ADC pin. There is firmware scaling to make the input appear to be a 1:1 input but this is rendered incorrect if an external voltage divider is used. For a 3.3V input this results in a resolution of 0.0129V per step or 0.002V per step at the pin. Raising the voltage above 3.3V resulted in a reported value of 255 from the ADC.

Input AD1 similarly is also not a true 1:4 input as stated in the manual. It also uses an internal 1:6 divider and uses software scaling to make it appear to be 1:4. Another inconsistency with AD1 is that being hardwired to the receiver power supply, it should theoretically be capable of measuring up to the maximum supply voltage, specified as being 10V. Curiously the manual states the maximum readable voltage on AD1 is 13.2V. In my measurements I found that I was unable to measure above 9.0V, corresponding to a reported value of 174 from the ADC. This also corresponded with an input voltage of 1.5V on the ADC pin. Checking the datasheet for the MSP430F2232 there are two possible maximum input voltage limits for an analog input that are selectable in the firmware: 1.5V or 2.5V. If FrSky have programmed for 1.5V, that would explain why I could not read above this value on AD1. If not, I am unsure why I couldn't measure above this point. With a reading of 9.0V at full scale (174) this gives a resolution of 0.052V per step or 0.008V at the pin.

If anyone decides to disable the internal dividers, be aware that you cannot read above 1.5V on AD1 or 0.55V on AD2 and should scale your inputs accordingly. Personally, I would rather see firmware in the D6FR that gave a reading of 0-255 for an input at the pin of 0-1.5V but seeing as it has been stated that firmware updates are not possible on this unit I guess it is unlikely.

While this may or may not concern a lot of users, I believe that potential customers should be aware that their existing tools and setups may not work with the D6FR without some modifications. The Flytron unit is one example of this.

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