How accurate the cpu cloc frequency on the various esp8266 modules? Two issues - how accurate are the crystals on the boards, and how is 80 Mhz (or 160 Mhz) derived from the 26 Mhz crystal oscillators? The easiest way that I can see is to divide the 26Mhz by 13 (to reach 2.0 Mhz) and then use a PLL (or moral equivalent) to multiply that frequency by either 40 or 80 to reach 80 or 160 Mhz. I know that this is achievable (Microchip does it with some of their PIC24 and dsPIC33 parts), but I wonder how must jitter there is and how expensive in chip resources (or not). And in any case, 26Mhz seems to me to be an odd choice for a crystal frequency, but maybe they are used in great volume somewhere else.