CC2500 power issue (formerly 'I need a Rx...')

Thank you very much for this information.
Since ThomasDB said to be using the same module as you are, he might have caught a broken/defective module.
So I guess it is safe to install the shielded module from BG.

But has anybody made experience with the other, even cheaper module available from BG ( http://www.banggood.com/CC2500-PCBA-2_4GHz-Wireless-Transceiver-Module-p-922598.html )
According to their specs, this one is said to have 'Maximum Output Power : +22dBm 200mW'
(which is not correct by the way, since 200mW equals to +23dBm)

Xoneoo, did you measure only the current through the CC2500 ?

Just for some interesting information when using the cc2500 module in a Devo 12s. The current drawn, even at the lowest setting of 100uw, was over 100ma, causing all the analog channels to have a 3 - 5 point deviation from the stick location. The fix for this was to install a separate power supply to feed the module with a common ground to the main board. At a full power of 150mw, the cc2500 was drawing around 170ma. I confirmed this with a second module that is installed in a Devo 7e.

Hi Storm can you check your devo 7 battery voltage, I have also strange behviour when using the frsky protocol and cc2500 that voltage displayed very instable and fluctuating. use same cheap banggood module.

I have already replaced the regulator in my devo 7e for this one:
www.pololu.com/product/2122

As the internal regulator became very hot, but still voltage in my devo display is instable, I have set emty tx bat alarm at 4.2volt, but it is often triggered during flying when battery is stil 4.5 4.6v.

And what is very strange even at 100uW setting, I still have this fluctuation behavior, but it is bit less.
Maybe need to place big capacitor at the modules?

I will check my 7e battery voltage later when I get home. I tested the 7e out last night with the cc2500 module and did not have nay issues with the battery fluctuating or any noticeable problems with it. I will pay closer attention tonight when I get home.

What version of deviation are you using? Have you checked for any shorts that might be causing the power supply to heat up?

latest nightly build, please touch the tiny stock regulator between the two big capacitors on the board it becomes very hot....
anyway I have with c2500 jumpy fluctution is my tx voltage displayed...

Fernandez wrote: latest nightly build, please touch the tiny stock regulator between the two big capacitors on the board it becomes very hot....
anyway I have with c2500 jumpy fluctution is my tx voltage displayed...

For testing set "Frysky-V8" on the model menu and the fluctuation is gone...
This behavior has been discused more than once.

Hey Amax, I checked and you are rightchanging to FrSky-V8 problem disappeared.

I never seen the discussion and the solution for this, for me this is a pretty serious bug, voltage display of txmust be reliable.....

Is this software implementation/bug, i think it is?
Or is it based on hardware, we need add some power filter capacitor close to the cc2500 VCC etc

Even though I don't have another tx than the 7e to compare, I am would say, it's hardware related.

A weak power supply (standard LDO is a 100mA type which has the maximum at 300mA peak) and
a powerfull RF module is discovered by this symptom.
My measurement for the whole tx (CC2500) is 196 to 207 mA and the fluctuation between 0.06 and 0.11V.
BTW, you will find this with the Walkera and the DSMX protocol as well.
That's one reason I am running my tx only at 100mW max.

But,..I am wondering why this is a problem on your tx, because I remember you changed the LDO.

I took a closer look and paid attention to the battery reading and it does fluctuate when using frsky. I plan on opening it back up and seeing what I can do to replace the on board 3.3 v supply. The package should be easy to replace it.

When I find a replacement that works I will post it here.

As mentioned I have already replaced the regulator in my devo 7e for this one:
www.pololu.com/product/2122

It is a step up/down type meaning even the battery below 3.3V can still work. And worksperfectly fineI tested on variable psu, Idid unsolder the miniature tock ldo onboard.


But still the devo 7e voltage fluctuates on some protocols such as worst case on Frsky, changing to Frsky V8 protocol problem is gone.
When using Frsky protocol even atlowestpower setting fluctuations are still therebut itis a bit lessvissible,but still there.

Hi guys I really do not understand it during flying, I really get annoying battery low messages form Tx when using Frsky telemetry on devo 7e.

TheTx voltage being displayed is just incorrect, it is too low, 0.3-0.4V and next to that it is fluctuating hugely. Basically a battery of 4.68V displayed with bayang protocol, give me when flying with Frsky beeping and even messages that batt volt too low to save. (the tx stillworks fine,as with polulo I am sure 3V3 always is ok, even when feeding with voltage below 3V3, stepup/down reg)

When set power level of Frsky to minimum setting 100uW, even then the voltage displayed on devo 7e is incorrect.

this is very annoying and would like to get the explanation as is potentially very dangerous problem.....

@Fernandez
What kind of battery is in your Devo?

Brand new eneloop pro but also alkaline same issue, it is not the battery.


Do we have a schematic of the devo 7e? Where is the measurement taken? Can it be interferenc on the pick up line? Why only some protocol have the issue worst is frsky...... A buffer capacitor at tx module would that help?

Anyone know if problem is also present on other tx such asdevo 10?

Did some measurements with bench power supply connected to my devo 7e

Frsky 5V (fluctuating voltage readout, current fluctuation)
100uW 146 mA
10mW 188mA
150mW 220mA

Frsky-D8 5V (stable voltage readout)
100uW 161 mA
10mW 235mA
150mW 292mA

Bayang nrf module 5V
100uW 79mA
10mW 81mA
150mW 92ma

DSM2 5V
100uW 98mA
10mW 102mA
150mW 125mA

The question is what current do you have on the cc2500? Is it correct that it is really fluctuating current? I would assume switching of PA on module is very vast so not visible as current fluctuation?

Do we have the schematic of devo 7e, can we put capacitor at the voltage sense line?

No schematic that I know if, just a little hardware info. . PB posted previously that he determined the voltage transfer equations by measurement.

Not sure where I got the attached diagram of the devo10 power input circuit, but it does not correspond to the measurements (the voltage divider doesn't match up). If the part about the FET switch is correct, there will be some voltage drop there that varies with current that will affect the measurement. Would be interesting to know the voltage drop there at various loads. After looking up the FET there shouldn't be any appreciable voltage drop there if the drive circuitry is working correctly.

Checking the ADC readings on my devo 10 with known voltages gives a constant about 6% different than what's in the code, which is about the difference between the screen and my battery. I think there was also a post about possibly making the constant adjustable by config file so people could compensate for differences in their own radios. More complicated would be a calibration step.

I'm not sure the current equation is the best. Seems to me if finding the constant by measurement then the curve fit should force the intercept to zero (the VOLTAGE_OFFSET should always be 0) since 0V into the ADC will give a count of 0.

Possibly a 10uF at C4, could help smooth out this fluctuation effect when using Frsky protocol, any one know Volt measurement point for devo 7e ?

Indeed the real voltage in my devo 7e and displayed oltage are also off, by 0.1 Volt

I connected a 10uF capacitor to the ADC pin of voltage sense line.

I coould see the Capacitor was working as when switching on Tx, it start directly with a vbat low alarm, then voltage level displayed now rises slowly (charging of the capcitor)and it takes several seconds to display up to full pack voltage.
But surprisingly the fluctuation on voltage displayed on the 7e remains the same.
Even with the capacitor still the voltage ws fluctuating excactely same manner......