ERSKYTx (was ersky9x) Questions

[MikeB]

Doesn't look like we can get the pins free for the quadrature encoder hardware, we need PC23 and PC24, but the PPM-in would then need to be on PC29, or PA0, PA15, PA26. These are the available capture inputs on the chip.

Mike.

[SkyNorth]

To measure the current we need a opamp .

I'm trying to find one that is not $1.50 in a mini package.

I will look at encoder inputs.... These are for a shaft encoder ....
I didn't think a high speed interface was needed ....but If I can , I will hook in.

-Brent

[MikeB]

Agreed we don't need high speed, just thought it would be nice if we just had to read a counter to get the encoder position. It's probably not worth the effort to frre the pins, the software can do it.

Re the op-amp, have you looked at Microchip, MCP601/MCP602. They are cheap, CMOS, rail-to-rail devices, and even come in 5 pin SOT23 size. Only £0.30 in ten off.

Mike.

[SkyNorth]

just looking at a MCP6L01 opamp from Microchip
nice and small , $0.21
-Brent

[SkyNorth]

I can free up the A/D , but if the "sense element" is in the Ground line , we wont be measuring the RF Module current , unless it is plugged into
the board.
If We measure the high side , then we get the whole current draw.
Also to do this I need the room the RTC chip is taking up

-Brent

[MikeB]

If you have put the 32kHz xtal on the CPU, I don't think the RTC chip is needed.
We do want to measure the whole current draw if possible.

Mike.

[SkyNorth]

I just found MAX4372.
It costs $1.50 , but is a very simple and small solution.
Also found ZXCT1009 , which is under a buck.

-Brent

[MikeB]

Either looks good to me, the ZXCT1009 may be the better choice. We can provide a calibration option in the software to get the best accuracy.

Mike.

[SkyNorth]

The ZXCT1009 looks very nice , simple to use ..
Can I get away with driving the A/D directly?

-Brent

[MikeB]

I would think so. How about setting it up to give an A2D voltage from 0 to 2.5V, but putting a 3.3V zener in parallel with the resistor in the current output of the ZXCT1009. We have enough resolution that 2.5V will give an accurate enough reading, and the zener should not leak too much current at 2.5V, however the processor input is protected is the current from the ZXCT1009 goes too high.

Mike.

[SkyNorth]

Doing the Math...

Using a sense resistor of 0.1R at 1A load gives 100mV

With a 2.7K load resistor , this will give a output of 2.7V

The shunt would have to be 1 Watt.

I could use a 27K , and a 0.01R shunt , but would the A/D load it too much?

-Brent

[MikeB]

I would hope we don't get to 1A Even at 1A, power is IxIxR = 1x1x0.1 = 0.1 watt, not 1 watt, so a 0.1 watt shunt should be OK.

We can handle high input impedances in the A2D, it has a programmable tracking time for this purpose. I actually need to do the calculations on this for best performance on all the A2D inputs, I've guessed values so far just to get it working.

Even with a large value load resistor, if there is a small cap in parallel (we are measuring DC) this would lower the impedance seen by the A2D.

I would go for a lower shunt and a higher load if possible, just to keep the current used by this circuit lower.

Mike.

[SkyNorth]

Missed a decimal ...good thing it wasn't going to Mars...

I will use 27K (we use it already)

1 Amp was a max for sure ...some people are using some higher power RF stuff so I gave it a bit more room.
(We can always change it)

Here is what I have...the data sheet says to add the series resistor , for load dump protection ...
I would rather do this than add a new part (zener) to the BOM

Or there a 5V input protectors on the USB lines , I could use one of these.

current sense.pdf

(49.62 KiB) Downloaded 487 times

[MikeB]

Looks OK, the protection resistor could be a much larger value. I suggested a zener to protect the CPU from the sense voltage going to high due to a failure in the ZXCT1009. Rather than 100 ohms, R59 could be 10K. This will provide a significant current limit if the CPU pin protection diode is forward biased, due to over voltage, while only droping 1V (max) in normal operation. So with 2.7V across the sense resistor, 1V across this protection resistor and 1.5V dropout voltage of the ZXCT1009 (from one of the graphs on the datasheet), we have a minimum supply voltage of 5.2V.

Can't find a current limit in the CPU datasheet for an over voltage on a pin.

Mike.

[ShowMaster]

"There is a 1N4002 diode as reverse polarity protection , and a 1/2 ohm resistor in the power line ... Resistor should smoke out
and open if there is a reverse polarity.

One worry is that some of these batteries , can deliver 100's of AMPs in a short circuit , This could vapourize the diode junction and open it , in most cases the
diode would short out, but if it opened it wont do much good.

-Brent"

What's wrong with a good old fashioned fuse instead of the 1/2 ohm (fusible?) resistor?
Just a thought,

Clive

Why not a single Schottky diode in series with the boards input. The drop would be ~.3 volts I think and the reverse leakage current should no damage any components? The voltage drop would have to be accounted for in the meter and alarm circuits. The bridge is a good idea but the idea is to be aware of polarity when ever possible and the diode is just insurance. I believe a 1A Schottky is available in a 1/4 watt sized package or SMD size?
SM

[SkyNorth]

I will add a series diode to the input.

The bridge sounds like a good Idea ,, but will drain the battery over time , due to leakage currents.
Also it needs to connect directly to the battery.

-Brent

[SkyNorth]

Would it not be better if there was a menu selection for the trainer port/mode.? - RF on / RF OFF

also the trainer port power switch will bypass the current sensor (and the power switch) when plugged in.

-Brent

[MikeB]

Can we switch the RF on and off? If in trainer mode, does the backlight light up? If not, we probably know the current drain so can allow for it.

Mike.

[MikeB]

Here's something interesting the new board could do. The analog to digital inputs have optional scaling and offset. Now the 9x has special pots that give the full voltage swing over the stick movement, but other pots and sticks give a lesser voltage range. No problem, just move the A2D offset to the middle of the voltage range, and scale the movement up by 2 or 4. Oh dear, another menu option .

Mike.

[Rob Thomson]

Does that mean that you could use 'regular' pots in the event that the stock ones fail?

[MikeB]

Should be possible. Pots often have a full rotation of 300 degrees, the sticks only 60 degrees, so we only have a fifth of the voltage. But, on the ersky9x board, we can scale this by 4 to get four fifths of the voltage, and we have 4 times the resolution (12 bit not 10 bit) to measure this with.
The offset moves the measurement centre point to half the supply voltage, so just have to adjust the pot centre to be at the same place.

Mike.

[SkyNorth]

We Cant turn the power off to the RF , but we can choose not to send a PPM signal to it.
So its power draw would be low.

It just seems there is a lot of problems with trainers....

=Brent

[MikeB]

No problem with turning the PPM to the RF module off if in trainer mode, and we have separated PPM to the RF and PPM in and PPM out to the jack so we have lots of options.

General to all, the developing software is now on googlecode, lots still to do, but its looking good so far. Just getting the menu system up and running.

Mike.

[wallaguest1]

some modules like the rmilec uhf does transmit at 2 watts power, i just comment it in case its something that can help in the development

[SkyNorth]

Here is the updated board.

The board now has a current sensor , to measure the system draw on the Battery.
Series Schottky diode for Reverse polartity protection. as well as a polatrity protection diode.
The ARM chip now has a 32khz crystal for RTC time.
There is still a spot for a RTC and battery , if needed in the future.
-Brent

ERSKY9x_RevB_PCB.pdf

(260.2 KiB) Downloaded 441 times

[Rafit]

We are currently doing a Beta run of 25 boards , there are a couple left ...price will be $85 + shipping.

I will write up a Spec sheet on the new board...

-Brent

Is there any possibility to buy one of those "25 board"? It can be an interesting challenge...

[SkyNorth]

Yes,
There are still a couple of boards unspoken for.

-Brent

[Reacher10]

I have one of my 9x's set up with frsky telemetry. What would I need to do to connect it to the ersky board?

My guess is I can get rid of the rs232 board I have in there now and use the one thats on the ersky board?
If this is correct then won't I have more connections to make to the main board then there is now? I'm not
up on all the connection type stuff.

Brent, your bag will be in the mail in the morning. I'll send you a pm on RCG.

[MikeB]

There are 2, 3-pin headers with RS232 level serial ports. One of these will be configured to connect to the FrSky telemetry data.

Mike.

[SkyNorth]

There are also external 2.5mm stereo connectors that can be accessed by drilling a hole in the case bottom.
This way a simple 3 wire cable can plug in to the back of the RF module provide telemetry.

Or, as Mike said you can also use the internal connections , if you are installing the internal RF module.

-Brent