Clocks and Power Manager

3.8.3Driving the Crystal Pins from an External Clock Source

The information in this section is provided as a guideline. The electrical specifications for the crystal oscillator pins are in Intel® PXA255 Processor Electrical, Mechanical, and Thermal Specification.

A 3.6864 MHz crystal must be connected between the PXTAL and PEXTAL pins. A 32.768 kHz crystal is normally connected between the TXTAL and TEXTAL pins. This configuration gives the lowest overall power consumption because the crystal’s resonant nature provides better power efficiency than an external source that drives the crystal pins. Some applications have other clock sources of the same frequency and can reduce overall cost by driving the crystal pins externally.

Refer to the Oscillator Electrical Specifications in the Intel® PXA255 Processor Design Guide for more information.

Note: No external capacitors are required.

3.8.4Noise Coupling Between Driven Crystal Pins and a Crystal Oscillator

The two pairs of crystal pins are located near each other on the processor. When crystal oscillators are connected to the pins, this proximity leads to low signal swings and slow edges that result in limited noise coupling between the pins. If one of the crystal oscillators is replaced by an independent signal source and the other is not, the noise coupling may increase. To limit this effect, reduce the slew rate on the pins driven by the independent source.

3.9Clocks and Power Manager Register Summary

3.9.1Clocks Manager Register Locations

Table 3-26shows the registers associated with the clocks manager and the physical addresses used to access them.

Table 3-26. Clocks Manager Register Summary

Address

Name

Description

 

 

 

0x4130_0000

CCCR

Core Clock Configuration Register

 

 

 

0x4130_0004

CKEN

Clock Enable Register

 

 

 

0x4130_0008

OSCC

Oscillator Configuration Register

 

 

 

3.9.2Power Manager Register Summary

Table 3-27shows the registers associated with the PM and the physical addresses used to access them.

Intel® PXA255 Processor Developer’s Manual

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Intel PXA255 manual Clocks and Power Manager Register Summary, Driving the Crystal Pins from an External Clock Source