Description

The BCM283X GPU has control over the frequencies of the various clocks that feed the hardware. Of these the most important are the ARM clock, which feeds the CPUs and the CORE clock which feeds most of the peripherals. On booting, the HAL reads the current, minimum and maximum values of these clocks and stores them in global variables. The UART and EMMC clocks are also read and recorded. The ARM and CORE clocks are then set to either the maximum values, or values set in the HAL configuration (CYGHWR_HAL_ARM_CORTEXA_BCM283X_ARM_FREQ and CYGHWR_HAL_ARM_CORTEXA_BCM283X_CORE_FREQ).

eCos does not contain any support for dynamic frequency management in the same way that Linux does. The CPUs run at a single frequency throughout. Normally this is the maximum frequency permitted without overclocking. A different frequency may be selected in the configuration, or it may be set at runtime by calling hal_bcm283x_set_freqencies().

The function hal_bcm283x_set_freqencies() updates either or both the ARM or CORE clock frequencies. The frequencies are given in Hz, and if zero are not changed. Values that lie below the minimum frequency for the clock are increased to the minimum and those above the maximum are reduced to the maximum. Other clock-related values, such as the mini-UART baud rate divider (which divides the CORE frequency), may also be changed. Finally, the curent values of the main clocks in the system are read back from the GPU to ensure that the correct values are used.