- Add a decoupling capacitor (0.1uF) close to the power pin on the AVR.
- Pull-up resistor on the AVR reset line (where the serial reset coupling capacitor goes to pin 1)
Good points! Also added a reset button, just in case. Pulling the power would probably work too, but a reset button is easy enough.
- What's the 600 ohm resistor on the output of the voltage follower for? What's its function?
It's in the datasheet for the LT1006 when used as a voltage follower/buffer.
http://cds.linear.com/docs/en/datasheet/1006fa.pdf (page 9) I'd previously asked about it here, and the thought was to leave it. Largely because the input resistance for the OCXO is listed as >9k, so the opamp isn't going to respond very fast without that resistor there. Probably doesn't NEED to respond very fast, but I figured since it's in the datasheet.
- Do you need a crystal on the microcontroller? Or are you using something like internal RC oscillator?
- Add a power LED or reset button maybe, but those are really just things that are "to your taste".
- Add a decoupling capacitor (0.1uF to ground) on the AVR's AREF pin.
- You will need a common ground going to the GPS board, not just Tx and Rx
- You may want to consider providing power from this board to the GPS board too, so everything is powered in one place.
-Crystal isn't needed, as I'm using the internal 8Mhz RC oscillator. Unit doesn't even need 8Mhz. =)
-LED for power will be supplied by the PLL board (it's got a 1PPS LED and power LED. LCD will also function as a sort of power LED.
-Decoupling the AREF I hadn't heard, but makes sense.
-Power and ground I forgot for the PLL board. Added those to this revision. The PLL board then has the connections for power and ground to the GPS board. Technically the TX/RX for the GPS come off the PLL board's MAX233, which gets it's data from the GPS board, so there shouldn't be any issues there.
- Unless you can rely on the user always using a centre-positive regulated 5V plugpack, don't just use a DC jack directly feeding your microcontroller and GPS etc from the external power supply without including a voltage regulator or protection circuit of some sort on board.
Regulator would suck because it would require more than 5V, and I'd like to keep things simple. I hate doing it (because I think it's lazy), but I've added a diode back biased across the power/gnd so if a center negative pack is used, the diode will keep things safe. If you have a more elegant way, that doesn't require more than a 5V dc input, I'm all ears (eyes?).
Okay, think I got everything. Also added a button I forgot that toggles the data on the LCD, as well as activates the backlight (single press, activate backlight, press again, change data). Also, if held, the button will turn on/off the OCXO. I added hardware to debounce the switch... pretty sure it's correct. I might move the pin it's on and stick it on an INT pin, but I'm not sure on that. I might also wire the 1PPS signal to the INT pin so I can sleep between updates.