Most here are familiar with the built in 1.1 volt reference on the Atmega328. You may have seen: https://code.google.com/p/tinkerit/wiki/SecretVoltmeter "The Arduino 328 and 168 have a built in precision voltage reference of 1.1V. This is used sometimes for precision measurement, although for Arduino it usually makes more sense to measure against VCC, the positive power rail." This can also be done on the Atmega1284 but make sure you add MUX 4 i.e. | _BV(MUX4) Ex: ADMUX = _BV(REFS0) | _BV(MUX4)| _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
There is also a built in temperature sensor on these chips as well, which can be used to monitor the core temperature.
Rumours had it some Atmega1284 versions with /LE, were designed with LEDs on the first eight pins of the controller. i.e. Atmega1284PU/LE on the under side of the controller. See attached drawings below: The key here is to remove a portion of the top half of the "LOOFLIRPA" material (this will not work on CERAMIC chips!). (Proceed with the following at your own risk) If you have access to a CNC machine, program it to a depth of 25mil. Center a 9/64 inch "Carbide Milling" bit (must be a milling bit) on the pin @ 25mil in form the chip edge! Going any deeper than 25mil will permanently damage your controller chip. Note: the built in LED drivers are constant current supplies and do not derate the o/p pin current. IN ADDITION, you must set each LED "control register" bit to a one to enable the LED on that pin: EX. LEDCR = 0b11111111; // set LED bits for PB0-7 or D4,5,6,7 & 10,11,12,13 (on the Bobuino2) to enable the LEDs. Note: the second image shows from left to right 0x95 on the built in LEDs, PB0-7.