SPRUIE9D May 2017 – May 2024 DRA74P , DRA75P , DRA76P , DRA77P
The DPLL recalibration applies only to type-A DPLLs. Each time the DPLL is reset or performs a lock sequence (following a change in the value of multiplier M or divider N), it performs a recalibration of the output frequency, based on voltage and temperature conditions. In lock mode, the DPLL maintains a steady lock frequency output by compensating for voltage and temperature changes within a certain range. However, if the voltage or temperature drifts outside the range or shows a significant or fast change, the DPLL may not be able to track and compensate it. It would need a recalibration, which is signaled by assertion of a recalibration flag.
During recalibration, the DPLL loses lock and output clock switches to the bypass clock.
The DPLL can automatically start recalibration when the recalibration flag is asserted, or recalibration can be triggered by software control. The trigger setting of the recalibration can be configured by the corresponding registers of the DPLL in the PRCM module. The software-controlled recalibration mode is selected by default.
Software-controlled recalibration: The DPLL continues its tracking mechanism as long as the recalibration is not triggered by software (that is, by enabling the recalibration-enable control parameter). If the DPLL reaches upper or lower bounds of the DCO control code and software has still not triggered recalibration, the DPLL stops its tracking mechanism. The output clock remains active, but frequency and jitter are not ensured to meet the requirement.
Automatic recalibration: The DPLL immediately starts the recalibration as soon as the recalibration flag is asserted.
Automatic recalibration of the DPLL can start at any time. While relocking, the DPLL switches to bypass mode, which introduces a frequency change. For modules that are sensitive to frequency change while operating, this can introduce operational instability. For example, the external memory EMIF controller is sensitive to a frequency change on the DPLL because its embedded DLL relocks on a frequency change. Any EMIF access during this DLL relock period can be corrupted. It is, therefore, important to stall EMIF access during DPLL recalibration.
To allow software to recalibrate the DPLL at the correct time depending on the device activity, the PRCM module can generate a wake-up event on the processor power domain, followed by an interrupt on the processor subsystem when the DPLL recalibration flag is asserted.
Table 3-46 lists the DPLL recalibration and control parameters.
Parameter | Register | Description |
---|---|---|
Recalibration-enable control | CM_CLKMODE_DPLL_<module>[8] DPLL_DRIFTGUARD_EN | Enable/disable the DPLL automatic recalibration feature. |
Recalibration-interrupt mask control | PRM_IRQENABLE_MPU PRM_IRQENABLE_MPU_2 PRM_IRQENABLE_IPU1 PRM_IRQENABLE_IPU2 PRM_IRQENABLE_DSP1 PRM_IRQENABLE_DSP2 | Mask/unmask the DPLL recalibration interrupt to processor. |
Recalibration-interrupt status | PRM_IRQSTATUS_MPU PRM_IRQSTATUS_MPU_2 PRM_IRQSTATUS_IPU1 PRM_IRQSTATUS_IPU2 PRM_IRQSTATUS_DSP1 PRM_IRQSTATUS_DSP2 | Status of the DPLL recalibration interrupt to processor |