Uploading with GPS Attached
Some people have been having issues uploading to their ArduPilot while the GPS is connected. I did some investigating today to determine where the issue may be.
I did a quick simulation of the circuit that turns the GPS on and off. This is a rough approximation, using a 650 ohm resistor to approximate the load of the GPS.
The circuit:
Results - Voltage across the load as a function of the voltage applied to the base resistor:
The minimum output high voltage specified by the ATmega datasheet is 4.2 V. So, it seems like it some situations the transistor might not actually turn off the GPS. I measured my board and found the voltage at the base resistor to be about 4.33 V, and according to the above plot, this means that my GPS should turn off, which in fact it does.
Comments?
I did a quick simulation of the circuit that turns the GPS on and off. This is a rough approximation, using a 650 ohm resistor to approximate the load of the GPS.
The circuit:
Results - Voltage across the load as a function of the voltage applied to the base resistor:
The minimum output high voltage specified by the ATmega datasheet is 4.2 V. So, it seems like it some situations the transistor might not actually turn off the GPS. I measured my board and found the voltage at the base resistor to be about 4.33 V, and according to the above plot, this means that my GPS should turn off, which in fact it does.
Comments?
Replies to This Discussion
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ModeratorPermalink Reply by Thomas J Coyle III on -
Damon,
I got the same voltage reading on the base of Q1 that you did indicating that the GPS should be off during the up load.
I believe that root cause of most of the upload problems with the GPS connected had to do with resetting the Atmega processor using the DTR/RTS control signals. The Sparkfun FTDI board uses the DTR signal while the Adafruit FTDI cable uses the RTS signal. The Configuration Utility program has be modified to use both the DTR and RTS signals to reset the processor so that the bootloader can keep Q1 turned off and begin the upload without the existing program trying to take control and keep Q1 on.
Regards,
TCIII
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Permalink Reply by Damon Pipenberg on -
Thomas,
The config utility has been modified to do this, see my post here. However, some can still not upload with the GPS connected, so I'm afraid the issue isn't quite solved yet. I don't actually have the problem, so it is difficult for me to troubleshoot it. This is just a guess at a possible issue. It seems that ~100mV of safety room between having Q1 on or off is cutting it pretty close.
I saw in another thread that you thought that pin 6 of the EM-406 might be an issue. Did you determine if that was an issue or not?
Damon
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ModeratorPermalink Reply by Thomas J Coyle III on -
Damon,
Unfortunately the termination of pin 6, according to Jordi, is not an issue in relation to the upload problem.
However, I did add a 10k pullup resistor connected to VCC to the base of Q1 and it did not help to keep Q1 cutoff until I updated the FTDI drivers.
Regards,
TCIII
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