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?
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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.
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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