2.14.1 High-level clock gating

High-level Clock Gating (HCG) is a mechanism supported by the PCCB that notifies the SoC when the CCN-502 is inactive. HCG enables an external SoC clock control unit, the External Clock Controller (ExtCC), to stop the CCN-502 GCLK0 clock inputs.

The CCN-502 includes a Q-Channel interface that enables the CCN-502 and the SoC to communicate to achieve HCG functionality through the PCCB. See the AMBA® Low Power Interface Specification, ARM® Q-Channel and P-Channel Interfaces for more information.

External clock controller

This section describes the external clock controller.

The following figure shows an example of how the ExtCC controls the clock gating flow. This example clock gating sequence begins and ends with the Q-Channel in either of the following states:

  • Quiescent state (Q_STOPPED), where QREQn and QACCEPTn are asserted.
  • Active state (Q_RUN), where QREQn and QACCEPTn are deasserted.
Figure 2-13 Clock gating control using ExtCC
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The requirements of the ExtCC are as follows:

  • It must supply a clock to the CCN-502 when the Q-Channel is in any state other than Q_STOPPED.
  • The ExtCC can either choose to gate the clock to the CCN-502 when the Q-Channel is in the Q_STOPPED state, or it can choose to run the clock at any time.
  • Although this manual does not describe the exact behavior of the ExtCC and its usage of QREQn in response to QACTIVE deassertion, the design of the ExtCC is likely to include a control loop with some hysteresis so that HCG is enabled when the system is inactive for long periods, but is not enabled for very short periods of inactivity. If the clocks are stopped in response to short periods of inactivity, performance of the CCN-502 can be negatively affected.
  • It is the responsibility of the SoC designer to fully control the clock management Q-Channel. If there is a requirement for a control or configuration bit to completely enable or disable HCG functionality, that register or bit must exist outside of the CCN-502. More specifically, the CCN-502 has no internal means of disabling HCG.
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