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 CL is within the range given in the table below. The exact value of CL can be found in the crystal data sheet. The capacitance of the external capacitors (CLEXT) can then be computed as follows:

Load Capacitance Equation:

CLOAD = ([CXIN + CLEXT] * [CXOUT + CLEXT]) / ([CXIN + CLEXT + CLEXT + CXOUT]) + CSTRAY

Where:

CLOAD = Crystal Mfg. CLOAD specification

CXIN = XOSC XIN pin data sheet specification

CXOUT = XOSC XOUT pin data sheet specification

CLEXT = Required external crystal load capacitor

CSTRAY (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 CXIN and CXOUT within 4 pF of each other, assume CXIN ~= CXOUT ~= CXTAL_EFF = ((CXIN+CXOUT) / 2) (Averaging CXIN and CXOUT will effect final calculated CLOAD value by less than 0.25 pF.)

The load capacitance equation can then be simplified as follows:

CLEXT = 2*CLOAD - CXTAL_EFF - 2*CSTRAY

Table 33-28. Crystal Oscillator Characteristics
Symbol Parameter Conditions Min. Typ. Max. Units
fOUT 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, CL = 100 pF
 XOSC.GAIN = 0 - - 5.6K Ω
f = 2 MHz, CL = 20 pF
 XOSC.GAIN = 0 - - 416
f = 4 MHz, CL = 20 pF
 XOSC.GAIN = 1 - - 243
f = 8 MHz, CL = 20 pF
 XOSC.GAIN = 2 - - 138
f = 16  MHz, CL = 20 pF
 XOSC.GAIN = 3 - - 66
f = 32 MHz, CL = 18 pF
 XOSC.GAIN = 4 - - 56
CXIN Parasitic capacitor load - 5.9 - pF
CXOUT Parasitic capacitor load - 3.2 - pF
Current Consumption f = 2 MHz, CL = 20 pF, XOSC.GAIN = 0, AGC off 27 65 87 μA
f = 2 MHz, CL = 20 pF, XOSC.GAIN = 0, AGC on 14 52 76
f = 4 MHz, CL = 20 pF, XOSC.GAIN = 1, AGC off 61 117 155
f = 4 MHz, CL = 20 pF, XOSC.GAIN = 1, AGC on 23 74 104
f = 8 MHz, CL = 20 pF, XOSC.GAIN = 2, AGC off 131 226 308
f = 8 MHz, CL = 20 pF, XOSC.GAIN = 2, AGC on 56 128 181
f = 16 MHz, CL = 20 pF, XOSC.GAIN = 3, AGC off 305 502 714
f = 16 MHz, CL = 20 pF, XOSC.GAIN = 3, AGC on 116 307 590
f = 32 MHz, CL = 18 pF, XOSC.GAIN = 4, AGC off 1031 1622 2260
f = 32 MHz, CL = 18 pF, XOSC.GAIN = 4, AGC on 278 615 1280
tSTARTUP Startup time f = 2 MHz, CL = 20 pF,
XOSC.GAIN = 0, ESR = 600Ω - 14K 48K cycles
f = 4 MHz, CL = 20 pF,
XOSC.GAIN = 1, ESR = 100Ω - 6800 19.5K
f = 8   MHz, CL = 20 pF,
XOSC.GAIN = 2, ESR = 35Ω - 5550 13K
f = 16   MHz, CL = 20 pF,
XOSC.GAIN = 3, ESR = 25Ω - 6750 14.5K
f = 32 MHz, CL = 18 pF,
XOSC.GAIN = 4, ESR = 40Ω - 5.3K 9.6K
Figure 33-4. Oscillator Connection