33.9.1.2 Crystal Oscillator Characteristics

The following table provides the characteristics for the oscillator when a crystal is connected between XIN and XOUT as shown in the figure Oscillator Connection. The user must choose a crystal oscillator where the crystal load capacitance C_Lis within the range given in the table below. The exact value of C_L can be found in the crystal data sheet. The capacitance of the external capacitors (C_LEXT) can then be computed as follows:

Load Capacitance Equation:

C_LOAD = ([C_XIN + C_LEXT] * [C_XOUT + C_LEXT]) / ([C_XIN + C_LEXT + C_LEXT + C_XOUT]) + C_STRAY

Where:

C_LOAD = Crystal Mfg. C_LOAD specification

C_XIN = XOSC XIN pin data sheet specification

C_XOUT = XOSC XOUT pin data sheet specification

C_LEXT = Required external crystal load capacitor

C_STRAY (Osc PCB capacitance) = 1.5 pF per 12.5 mm (0.5 inches) (TRACE W = 0.175 mm, H = 36 μm, T = 113 μm)

For C_XIN and C_XOUT within 4 pF of each other, assume C_XIN ~= C_XOUT ~= C_{XTAL_EFF} = ((C_XIN+C_XOUT) / 2) (Averaging C_XIN and C_XOUT will effect final calculated C_LOAD value by less than 0.25 pF.)

The load capacitance equation can then be simplified as follows:

C_LEXT = 2*C_LOAD - C_{XTAL_EFF} - 2*C_STRAY

Table 33-28. Crystal Oscillator Characteristics
Symbol	Parameter	Conditions	Min.	Typ.	Max.	Units
f_OUT	Crystal oscillator frequency	-	0.4	-	32	MHz
ESR	Crystal Equivalent Series Resistance Safety Factor = 3 The AGC doesn’t have any noticeable impact on these measurements.	f = 0.455 MHz, C_L = 100 pF  XOSC.GAIN = 0	-	-	5.6K	Ω
		f = 2 MHz, C_L = 20 pF  XOSC.GAIN = 0	-	-	416
		f = 4 MHz, C_L = 20 pF  XOSC.GAIN = 1	-	-	243
		f = 8 MHz, C_L = 20 pF  XOSC.GAIN = 2	-	-	138
		f = 16 MHz, C_L = 20 pF  XOSC.GAIN = 3	-	-	66
		f = 32 MHz, C_L = 18 pF  XOSC.GAIN = 4	-	-	56
C_XIN	Parasitic capacitor load		-	5.9	-	pF
C_XOUT	Parasitic capacitor load		-	3.2	-	pF
	Current Consumption	f = 2 MHz, C_L = 20 pF, XOSC.GAIN = 0, AGC off	27	65	87	μA
		f = 2 MHz, C_L = 20 pF, XOSC.GAIN = 0, AGC on	14	52	76
		f = 4 MHz, C_L = 20 pF, XOSC.GAIN = 1, AGC off	61	117	155
		f = 4 MHz, C_L = 20 pF, XOSC.GAIN = 1, AGC on	23	74	104
		f = 8 MHz, C_L = 20 pF, XOSC.GAIN = 2, AGC off	131	226	308
		f = 8 MHz, C_L= 20 pF, XOSC.GAIN = 2, AGC on	56	128	181
		f = 16 MHz, C_L = 20 pF, XOSC.GAIN = 3, AGC off	305	502	714
		f = 16 MHz, C_L = 20 pF, XOSC.GAIN = 3, AGC on	116	307	590
		f = 32 MHz, C_L = 18 pF, XOSC.GAIN = 4, AGC off	1031	1622	2260
		f = 32 MHz, C_L = 18 pF, XOSC.GAIN = 4, AGC on	278	615	1280
t_STARTUP	Startup time	f = 2 MHz, C_L = 20 pF, XOSC.GAIN = 0, ESR = 600Ω	-	14K	48K	cycles
		f = 4 MHz, C_L = 20 pF, XOSC.GAIN = 1, ESR = 100Ω	-	6800	19.5K
		f = 8 MHz, C_L = 20 pF, XOSC.GAIN = 2, ESR = 35Ω	-	5550	13K
		f = 16 MHz, C_L = 20 pF, XOSC.GAIN = 3, ESR = 25Ω	-	6750	14.5K
		f = 32 MHz, C_L = 18 pF, XOSC.GAIN = 4, ESR = 40Ω	-	5.3K	9.6K