SELF-CALIBRATION

Yet another feature of the LUB system is that space-hogging calibrations that only need to be run occasionally can be kept in the bootloader. In this example, there is no external crystal. Instead, the internal RC oscillator is calibrated against a real-time clock that provides a 4,096-Hz output signal. The target frequency is 7.37 MHz, which enables error-free data rate communication at 115,200 bps. After calibration, the value is stored in the DataFlash, where it can be read by the tinyloader and stored in the OSCCAL register at every start-up.

The sharp-eyed among you are probably wondering if you’ll be left with a dead system that can’t communicate if power fails while you’re writing this value. No! If the power fails, the ATmega88 AVR will still have the bootloader code loaded into its flash memory. At the next start-up, the tinyloader won’t see valid data from the computer. It will proceed to start the code in the ATmega88. This code, which is the bootloader, will calibrate the oscillator and store it. Now you’ll have a system that works. At that point you can finish working with the bootloader. Whether or not emulating a certain operating system—an operating system for which troubleshooting involves rebooting—is advantageous is up to you.