ben-wpan/atrf/ecn/ecn0002.txt

Add load capacitors to 16 MHz crystal (C14, C15)


The crystal has a specified load capacitance of 8 pF. The AT86RF230 has
an internal capacitor array that can be trimmed in 16 steps from 0 pF to
4.5 pF.

The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature
tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were
measured with the CNTR board, roughly at 20-25 C:

Board		Error		Meas. accuracy	C14/C15		Trim
		(f, ppm)	(ppm, nom.)	(pF)		(pF)
---------------	---------------	---------------	------------	--------
2, "orange"	+268		 99.6		NC		0
		+266		 99.5		NC		0
		+266		 40.0		NC		0
		+148		 99.8		NC		3.0
		+147		 99.7		NC		3.0
		+147		 40.0		NC		3.0
		+111		100.0		NC		4.5
		+110		 99.8		NC		4.5
		- 13		 99.9		15		0
		- 14		100.0		15		0
		+ 11		 99.9		12		0
		+ 11		 99.6		12		0
(cleanup)	+ 14		100.0		12		0
		+ 14		100.0		12		0

1, "red"	+263		 99.7		NC		0
		+263		100.0		NC		0
		+107		 99.3		NC		4.5
		+108		100.0		NC		4.5
		+ 24		 99.8		10		0
		+ 24		100.0		10		0
		-  9		 99.7		10		4.5
		-  9		100.0		10		4.5
(cleanup)	+ 14		100.0		12		0
		+ 14		100.0		12		0

(Multiple measurements to assess CNTR performance and drift. Note that
CNTR measurements are repeatable within 1 ppm, so the nominal accuracy
appears to be far too pessimistic.)


Conclusion: these measurements suggest that, combined with parasitic
capacitance, load capacitors of 12 pF make the crystal roughly half the
trim range faster than 16 MHz.