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CNTFRQ, Counter-timer Frequency

The CNTFRQ characteristics are:

Purpose

This register is provided so that software can discover the frequency of the system counter. The instance of the register in the CNTCTLBase frame must be programmed with this value as part of system initialization. The value of the register is not interpreted by hardware.

Configuration

The power domain of CNTFRQ is IMPLEMENTATION DEFINED.

On a reset of the reset domain in which an RW instance of this register is implemented, RW fields in the register reset to UNKNOWN values. The register is not affected by a reset of any other reset domain. For more information see 'Power and reset domains for the system level implementation of the Generic Timer' in Chapter I1 of the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile.

Attributes

CNTFRQ is a 32-bit register.

Field descriptions

The CNTFRQ bit assignments are:

313029282726252423222120191817161514131211109876543210
Clock frequency

Bits [31:0]

Clock frequency. Indicates the system counter clock frequency, in Hz.

This field resets to an architecturally UNKNOWN value.

Accessing the CNTFRQ

CNTFRQ must be implemented as an RW register in the CNTCTLBase frame.

In a system that recognizes two Security states, the instance of the register in the CNTCTLBase frame is only accessible by Secure accesses.

CNTFRQ can be implemented as a RO register in any implemented CNTBaseN frame, and in the corresponding CNTEL0BaseN frame.

'CNTCTLBase status and control fields for the CNTBaseN and CNTEL0BaseN frames' in Chapter I2 of the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile describes the status fields that identify whether a CNTBaseN frame is implemented, and for an implemented frame:

  • Whether the CNTBaseN frame has virtual timer capability.
  • Whether the corresponding CNTEL0BaseN frame is implemented.
  • For an implementation that recognizes two Security states, whether the CNTBaseN frame, and any corresponding CNTEL0BaseN frame, is accessible by Non-secure accesses.

For an implemented CNTBaseN frame:

  • CNTFRQ is accessible in that frame, as a RO register, if the value of CNTACR<n>.RFRQ is 1.
  • Otherwise, the CNTFRQ address in that frame is RAZ/WI.

For an implemented CNTEL0BaseN frame:

  • CNTFRQ is accessible as a RO register in that frame if both:
    • CNTFRQ is accessible in the corresponding CNTBaseN frame.
    • Either the value of CNTEL0ACR.EL0VCTEN is 1 or the value of CNTEL0ACR.EL0PCTEN is 1.
  • Otherwise, the CNTFRQ address in that frame is RAZ/WI.

CNTFRQ can be accessed through the memory-mapped interfaces:

ComponentFrameOffsetInstance
TimerCNTBaseN0x010CNTFRQ

Access on this interface is RO.

ComponentFrameOffsetInstance
TimerCNTEL0BaseN0x010CNTFRQ

Access on this interface is RO.

ComponentFrameOffsetInstance
TimerCNTCTLBase0x000CNTFRQ

Access on this interface is RO.



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