Line Frequency Measerment precision compliance with class 0.2s

The datasheet is based on perfect 24Mhz. 

enable  Dready out on cf4 and set Dready to 8Khz

ADE_SPI_WRITE(ADDR_WFB_CFG,0x0200,0x2); //set dready to 8khz
ADE_SPI_WRITE(ADDR_CONFIG1,0x000C,0x2); //enable cf4 to dready if you want to look at it

Measure Dready with a good freq counter I used HP53131A 

I measured on my board 7998.2276hz

Serial.println((7998.2276 * 65536.00) / (ADE_SPI_READ(ADDR_APERIOD,4)),5);

After using this value I found my freq measurement to be spot on. 

7998.2276 * 3000 = crystal freq  = 23994682.8hz

For high accuracy freq measurement you may need to verify the crystal freq.

my source will only go to .1hz resolution for my test 

you can also use your freq measurement to calculate the sample rate. 

if you 45 hz is perfect and you read 45.016 

8000 * 45/45.016  = 7997.156566

Using your data and 45hz as a correction value. There may be some rounding as I only had 3 digits.

Dave

One more thing to add is I would use an average of a few readings to improve accuracy. Zero crosses are used for the measurement and noise will cause some jitter in the freq measurement. The amount of average depends on you needed update rate of the line freq you require. 

Dave