32.12.1.2 Crystal Oscillator Characteristics

The following table describes 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. 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 32-42. 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 85 μA
f = 2 MHz, CL = 20 pF, XOSC.GAIN = 0, AGC on 14 52 73
f = 4MHz, CL = 20 pF, XOSC.GAIN = 1, AGC off 61 117 150
f = 4MHz, CL = 20 pF, XOSC.GAIN = 1, AGC on 23 74 100
f = 8 MHz, CL = 20 pF, XOSC.GAIN = 2, AGC off 131 226 296
f = 8 MHz, CL = 20 pF, XOSC.GAIN = 2, AGC on 56 128 172
f = 16 MHz, CL = 20 pF, XOSC.GAIN = 3, AGC off 305 502 687
f = 16 MHz, CL = 20 pF, XOSC.GAIN = 3, AGC on 116 307 552
f = 32 MHz, CL = 18 pF, XOSC.GAIN = 4, AGC off 1031 1622 2200
f = 32 MHz, CL = 18 pF, XOSC.GAIN = 4, AGC on 278 615 1200
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 32-6. Oscillator Connection