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Intel® PXA255 Processor Developer’s Manual 3-41
Clocks and Power Manager
3.8.3 Driving 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.
3.8.4 Noise 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.
3.9 Clocks and Power Manager Register Summary
3.9.1 Clocks Manager Register Locations
Table 3-26 shows the registers associated with the clocks manager and the physical addresses used
to access them.
3.9.2 Power Manager Register Summary
Table 3-27 shows the registers associated with the PM 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