11.6 Crystal Oscillator

The table below provides the crystal oscillator characteristics.

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.

The following figure shows the crystal oscillator schematic.

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 0.7 pF.
C_PCB is the ground-referenced parasitic capacitance of the printed circuit board (PCB) on the XIN and XOUT tracks.

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

The PLC protocol firmwares support propagation of the input clock signal through the TXRX1 pin, allowing fine adjustment of the Cx parameter.

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), therefore its value can be modified to obtain a drive level that complies with the requirements. As the value of R_S 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 required by the G3-PLC specification, the system clock tolerance must not exceed ±25 ppm. For the PRIME specification, the maximum allowable tolerance is ±50 ppm, while for the Meters and More specification, it is ±150 ppm. Regardless of the specification, Microchip recommends selecting a crystal with a frequency tolerance that is lower than the specified maximum. This is due to actual performance may be affected by production tolerances, temperature stability, and aging.