11.6 Crystal Oscillator

Table 11-8. 24 MHz Crystal Oscillator Characteristics
Parameter	Test Condition	Symbol	Rating			Unit
Parameter	Test Condition	Symbol	Min	Typ	Max	Unit
Crystal Oscillator frequency	Fundamental	X_tal	24			MHz
Internal parasitic capacitance	Between XIN and XOUT	C_PARA24M	0.6	0.7	0.8	pF
Start-up time		t_ON	—	—	1	ms
Drive level		P_ON	—	—	400	µW
Load capacitance		C_LOAD	4	—	18	pF

Important: Locate the crystal as close as possible to the XOUT and XIN pins.

Figure 11-1. 24 MHz Crystal Oscillator Schematic


C_X = 2 x (C_XTAL – C_PARA24M – C_PCB /2)

Where C_XTAL is the load capacitance of the crystal, C_PARA24M is the internal parasitic impedance of the oscillator, typically 0.7pF and C_PCB is the ground-referenced parasitic capacitance of the printed circuit board (PCB) on XIN and XOUT tracks.

Table 11-9 summarizes recommendations to be followed when choosing a crystal.

The drive level indicates power consumption by the crystal unit while the oscillation circuit works. Excessive drive level might cause unexpected change of frequency, crystal damage or shorter device lifetime. The drive level is given by the following formula:


$Drive Level = ESR \times I^{2}$

Where:

ESR is the equivalent series resistor (specified by the crystal manufacturer).
I is the current flowing through the crystal in RMS. If the waveform is sine wave or similar, the effective value is calculated by Ip-p/2√2.

To keep the drive level value inside the manufacturer range, it is required to limit the current that flows through the crystal. The total power dissipated by the crystal is proportional to R_S (see Figure 11-1), so its value can be modified to obtain a drive level that complies with the requirements. As the value of R_S also relates to the safety factor, there is a maximum value of R_S that maintains a good safety factor and at the same time keeps the drive level below the crystal manufacturer maximum specifications.

Table 11-9. Recommended Crystal Characteristics
Parameter	Symbol	Rating			Unit
Parameter	Symbol	Min	Typ	Max	Unit
Equivalent Series Resistor	ESR	—	—	100	Ω
Motional capacitance	C_M	2	—	3.2	fF
Shunt capacitance	C_SHUNT	—	—	1.3	pF
Frequency tolerance	—	—	10⁽¹⁾	25	ppm

Notes:

As a requirement of the G3-PLC specification, the System Clock tolerance shall be ±25 ppm maximum. In the case of PRIME specification, it shall be ±50 ppm maximum. In both cases, Microchip recommends choosing a crystal with a frequency tolerance below the specification requirement because it is dependent on production tolerance, temperature stability and age.