The chip's firmware is written in C, using the AVR-GCC toolkit which is available for free. Although all source code is open, the attached zip file as well contains the compiled hex file that can directly be flashed into the chip.
The code is documented generously and can be easily extended with additional functions. Only interface component is a single push button which, when pushed while powering up, brings the device into calibration mode.
In my prototype I chose to leave the button inside the enclosure.
Note that the software can work with any kind of AD 8307 circuity, with or without compensation network, at this or any other range of input voltages or slopes. This should give enough flexibility for other designs.
The calibration routine expects a signal of 0dbm and of -50dbm to be fed into the signal input and will compute the offset and slope values internally. When skipping the calibration, the chip will be initialized with values I found to work for my prototype.
If you have decided to add the compensation network, first turn the capacitor all the way out (minimizing its capacitance). Then calibrate the meter as described above with a 150 MHz signal from a signal generator. After that, feed a 0 dbm 450 MHz signal and turn the trim capacitor until the meter displays 0 dbm.
The easiest way to build the watt meter is by using SMD components directly on a single sided PCB where "islands" have been cut with a Dremel tool.
This construction method is fairly adequate for frequencies in the desired range (see picture below).
Be careful when using wired components when operating at higher frequencies than HF.
To avoid issues with RF from the CPU being seen by the 8307, I have separated the RF and CPU section through a piece of PCB soldered perpendicularly to the main board, passing through only the output and power line. The AVR supports doing the analog to digital conversion within a sleep cycle which would further reduce QRM should this become necessary. So far I have not seen the need for this. When the enclosure is open, it is sensitive to QRM generated by the PC though (most likely the USB port on the programmer).
Be careful when using external power supplies as those can introduce additional RF noise into the circuit. Either be sure to filter the incoming voltage or to use a battery instead (compare OZ2CPU's page).
As is, the controller uses it's internal 2.56V bandgap reference. If you choose to use VCC or another voltage instead, the configuration needs to be adapted.
Measuring against my reliable 8640B, I have not seen deviations beyond 0.5dbm over the frequency and power ranges from the expected values. When under 30 MHz, the deviation is within +/- 0.1dbm
So, for fairly little effort and cost, I am very happy with the result!