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USB Data Logger Firmware Improvements

by Nicholas Vinen

USB Data Logger firmware improvements

We recently made some improvements to the firmware for the USB Data Logger (December 2010 to February 2011) in addition to the memory card related fixes. These were alluded to in the errata published last month but here are some additional details.

Firstly, while tracking down a power supply problem, we noticed that when power was initially applied to the data logger (eg, when the battery is inserted), it often went into the USB bootloader mode even though pushbutton S2 was not being held down at the time. This was especially the case when the battery voltage was on the low side but sometimes happened even with higher battery voltages.

The reason for this is that the state of pushbutton S2 was being sensed using a digital input pin and S2 is multiplexed with the battery voltage sensing resistors. When S2 is pressed, pin AN4 of IC1 is pulled to ground and so if used as a digital input (RA5), it registers as low (zero). But when S2 is not pressed, it will only be pulled up to a maximum of half the battery voltage. If the battery is 3V then this is 1.5V.

But the datasheet says that for digital input pins with a TTL buffer (which includes RA5/AN4) and a 3.3V micro supply voltage, the minimum voltage to reliably read a pin as high is 1.625V. This is unlikely to be exceeded with any two cell AA battery. So the firmware would often read the state of S2 as being pressed even when it was not.

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