Data Sheet

122
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
When the APB clock for a module is not provided its registers cannot be read or written. The module can be re-enabled
later by writing the corresponding mask bit to one.
A module may be connected to several clock domains (for instance, AHB and APB), in which case it will have several
mask bits.
Note that clocks should only be switched off if it is certain that the module will not be used. Switching off the clock for the
NVM Controller (NVMCTRL) will cause a problem if the CPU needs to read from the flash memory. Switching off the
clock to the Power Manager (PM), which contains the mask registers, or the corresponding APBx bridge, will make it
impossible to write the mask registers again. In this case, they can only be re-enabled by a system reset.
15.6.2.7 Reset Controller
The latest reset cause is available in RCAUSE, and can be read during the application boot sequence in order to
determine proper action.
Table 15-1. Peripheral Clock Default State
Peripheral Clock Default State
CLK_PAC0_APB Enabled
CLK_PM_APB Enabled
CLK_SYSCTRL_APB Enabled
CLK_GCLK_APB Enabled
CLK_WDT_APB Enabled
CLK_RTC_APB Enabled
CLK_EIC_APB Enabled
CLK_PAC1_APB Enabled
CLK_DSU_APB Enabled
CLK_NVMCTRL_APB Enabled
CLK_PORT_APB Enabled
CLK_HMATRIX_APB Enabled
CLK_PAC2_APB Disabled
CLK_SERCOMx_APB Disabled
CLK_TCx_APB Disabled
CLK_ADC_APB Enabled
CLK_AC_APB Disabled
CLK_DAC_APB Disabled
CLK_PTC_APB Disabled
CLK_USB_APB Enabled
CLK_DMAC_APB Enabled
CLK_TCC_APB Disabled
CLK_I2S_APB Disabled