Koepel:
Are you sure that you have genuine MCP9808 chips ? Can you read that number on the chip ?
You replaced the MCP9808 with another, is that other one from the same batch from the same seller ?
...unbelievable. Well played, sir. I just looked at the chip under a magnifier...it's got a D2745 on the top line and a 639A3 on the bottom line. It seems that Digi-Key shipped a DS2745 in a MCP9808 labeled bag. I feel like an idiot.
I should've been more suspicious about this order because the .1uF 0805 bypass caps I ordered were on the invoice but not in the box so I ghetto bodged through hole caps from my parts bin onto this prototype. I've never had any trouble with Digi-Key, but this order has been a disaster.
Koepel:
Did you know that there is a bootloader which allows to use part of the Flash to store data ? With a small sketch, there might be 10 kbytes to store data.
I learned about this a while back but had forgot about this option. This is done via PROGMEM, right? This version is a prototype and I plan to expand this project to record additional parameters, so I think I'll need additional memory space anyway. Thanks for the reminder. I had only considered the onboard EEPROM.
Koepel:
A few notes about the schematic:
There is no diode from /RESET to VCC.
Do you plan to use /RESET and RX and TX to upload a sketch with a 100 nF capacitor from DTR to /RESET ?
Because of that capacitor, there could be a voltage peak on the /RESET, turning the /RESET in to High Voltage programming mode. That could corrupt the bootloader.
I upload my programs with an AVR ISP Mk2 via an AVR ISP header on the board, so all I've ever needed to do was include the 10k resistor to allow the ISP to pull RESET low for programming. The RX and TX pins connect to an external Serial-USB FTDI adapter that I use to download the data into a .csv file and chart in Excel. Do I need a diode or cap here?!? Never used this in the past and I don't use the software reset.
Koepel:
130 ohms for the Led ? Must it really be that bright ? Can you use a extra bright led instead of extra current ?
Valid point. I've had trouble seeing LED's in the bright sunlight while I'm hiking, so I kept the current high. Extra bright LED vs high current would be a wise tradeoff. Because this is battery powered and meant to run for over a week, I only allow the LED to blink when data has been recorded and when selected by turning a DIP switch on. I use this to see if it's still alive while on the trail 4 or 5 days in.
Koepel:
Running with the internal oscillator might cause trouble for the baudrate and other timing based protocols.
Hmmm. Hadn't considered this. Is this because the internal oscillators just aren't as precise?
So far, I've recorded date and time stamps every 5 minuets for about 10 days and was able to download from the EEPROM to a laptop with no trouble at any selected baud rate. Maybe I'm lucky.
Would you recommend an external oscillator when using timing based protocols in general? How would this impact the power consumption? I'm trying to keep this running at 1 MHz for power saving reasons.
Thanks so much for the inputs! I hadn't considered much of what you've pointed out and I really enjoy expanding my understanding.
Board layout attached for review. Photos were too large to attach.