there is documentation in the github manuals for the U1 usb device. It relies on a weird mathematical method of several combined binary strings from the hex code. Once sorted out its *relatively* straight forward to calculat eany frequency you want, to about 1.75MHz precision
Thanks for the information. Fancy math indeed! I wasn't able to "reverse engineer" the calculation (ie take a desired frequency and find the Hex value that I should use on the mining script) but I was able to calculate valid values for all the 4 digit Hex numbers. I found a lot that were supposed to be valid but they didn't work when using my U2 with BFGMiner. Maybe my math was wrong or maybe the U2 don't take all valid values in the formula. It's more probable my math was wrong, since I got some weird "valied" values, like 306.1429 or something like that. Will keep experimenting and create a list of valid HEX/frequency combinations.
FM
i threw out the paper i was working on, but the biggest factor is to determine your desired frequency and work backwards.
Final Freq (lets say its 300) = 25Mhz (this wont change) x [N/M] (M being 1, 2, or 4 based on the last hex value being a 0,1,or 2 respectively).
That makes calculating 'N' from the formula a bit easier. Once you do the method 2 or 3 times its easier to predict the hex code you will need. (for example, the 200-225 range is generally a x0yz with x being 4 or 5 in most of my work for that frequency range.
and using the U1 or U2 as a testbench is a great way to see if it will run and check if the average hashrate is what you expected.