DS20006554A datasheet(11/94 Pages) MICROCHIP

11 / 94 page

ZL30260-ZL30263

Data Sheet

DS20006554A

Note: the bits of the I2C address are as shown above by default but can be changed in the I2CA register. A

device’s I2C slave address can be set to any value during auto-configuration at power-up by writing the I2CA

5.3

Local Oscillator or Crystal

Section 5.3.1 describes how to connect an external oscillator and the required characteristics of the oscillator. Section

5.3.2 describes how to connect an external crystal to the on-chip crystal driver circuit and the required characteristics

of the crystal. The device does not require an external oscillator or crystal for operation.

5.3.1

External Oscillator

A signal from an external oscillator can be connected to the XA pin (XB must be left unconnected).

Table 8 specifies the range of possible frequencies for the XA input. To minimize jitter, the signal must be properly

terminated and must have very short trace length. A poorly terminated single-ended signal can greatly increase

output jitter, and long single-ended trace lengths are more susceptible to noise. When MCR2.XAB=10, XA is enabled

as a single-ended input.

While the stability of the external oscillator can be important, its absolute frequency accuracy is less important

because any known frequency inaccuracy of the oscillator can be compensated by adjusting the APLL's fractional

feedback divider value (AFBDIV) by ppb or ppm.

The jitter on output clock signals depends on the phase noise and frequency of the external oscillator. For the device

to operate with the lowest possible output jitter, the external oscillator should have the following characteristics:

•

Phase Jitter: less than 0.1ps RMS over the 12kHz to 5MHz integration band

•

Frequency: The higher the better, all else being equal

5.3.2

External Crystal and On-Chip Driver Circuit

The on-chip crystal driver circuit is designed to work with a fundamental mode, AT-cut crystal resonator. See Table

2 for recommended crystal specifications. To enable the crystal driver, set MCR2.XAB=01.

Figure 4 - Crystal Equivalent Circuit / Recommended Crystal Circuit

See Figure 4 for the crystal equivalent circuit and the recommended external component connections. The driver

circuit design includes configurable internal load capacitors. For a 10pF crystal the total capacitance on each of XA

and XB should be 2 x 10pF = 20pF. To achieve these loads without external capacitors, register field XACR3.XACAP

should be set to 20pF minus actual XA external board trace capacitance minus

XA’s minimum internal capacitance

of 6pF. For example, if external trace capacitance is 2pF then XACAP should be set to 20pF

– 2pF – 6pF = 12pF.

capacitance other than 10pF usually can be supported with only internal load capacitance. If the XACAP and XBCAP

fields do not have sufficient range for the application, capacitance can be increased by using external caps C1 and

C2.

Users should also note that on-chip capacitors are not nearly as accurate as discrete capacitors (which can have 1%

accuracy). If tight frequency accuracy is required for the crystal driver circuit then set XACAP and XBCAP both to 0

and choose appropriate C1 and C2 capacitors with 1% tolerance.

XTAL

Crystal

6pF + XACAP*1pF

(CL = 10pF)



6pF + XBCAP*1pF

(optional)

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