AVR Freaks

However, the ATmega32U4 on the custom PCB is flashed with the same code, it is not able to communicate to the battery gauge.

Can you successfully download any program?

Can you just simply flash an LED at 1 Hz?

Let's see what others say, but I do not like your micro Reset\ circuitry.

It appears that J13 is an ISP programming header.

The Reset\ signal from the programmer should connect directly to the Reset\ pin on the micro.

If the Reset\ signal is asserted, then the micro will go into its Reset Mode, and the I/O pins will all go into a high impedance input state.

Hence the GPIO_5 that feeds the Gate will float.

Ah, I see John already mentioned that.

But why qualify the programmer's Reset\ signal with the logic gate and a pin from the micro being Reset\ ?

I suspect that without a good reason for doing otherwise, I would have connected the programming Reset\ directly to the micro, and put a 200 ohm or whatever resistor in series with the Reset logic gate's output.

(Need to check its drive capability).

As mentioned check the USART signals and make sure that they aren't reversed.

Flash one of the LEDs 10 times at 1 Hz on start up to make sure the micro is running at the correct frequency, at least for now.

I'll never understand Active Low LEDs, but that has nothing to do with why the board isn't working.

As you have plenty of spare I/O pins, I would have brought one out to a "spare" LED just for testing and debugging.

You don't have to populate it if you don't want to, but the extra pads cost you nothing.

You can also use that I/O pin or another one, to a spare header, for hooking up an O'scope, either to time ISR's, or as a scope trigger when looking at other signals, etc.

I'm not a USB hardware guy.

But I'll just ask if you need / should have any voltage clamps on the USB data lines?

And/or do either of them need a pull-up to define the USB speed?

And, since it isn't readily clear to me from the schematic, is there a Ground connection from the micro to the USB Header's Ground pin?

JC   

I get it, it just bothers my old brain.

And I do understand that it makes absolutely no difference for the end user, or the hardware, or the software, (as long as the coder knows whether it is using reverse logic or not).

In the STK500 which also uses the reverse logic there is a driver transistor for each LED, and the driver lets the LED turn fully on regardless of the Vcc level for the micro.

In this case there is no benefit to the reverse logic, and I just like positive logic better.

I never was much good with PNP transistors, either...

I did some TTL design years ago.

I don't recall ever using DTL gates / chips.

That seems like a long time ago, (sigh).

JC