Datasheet

106
Atmel | SMART SAM D20 [DATASHEET]
Atmel-42129K–SAM-D20_datasheet–06/2014
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 Clock Failure Detector
This mechanism allows the main clock to be switched automatically to the safe OSC8M clock when the main clock
source is considered off. This may happen for instance when an external crystal oscillator is selected as the clock source
for the main clock and the crystal fails. The mechanism is to designed to detect, during a OSCULP32K clock period, at
least one rising edge of the main clock. If no rising edge is seen, the clock is considered failed.
The clock failure detector is enabled by writing a one to the Clock Failure Detector Enable bit in CTRL (CFDEN_CTRL).
Refer to CTRL for detailed information.
As soon as the Clock Failure Detector Enable bit (CTRL.CFDEN) is one, the clock failure detector (CFD) will monitor the
undivided main clock. When a clock failure is detected, the main clock automatically switches to the OSC8M clock and
the Clock Failure Detector flag in the interrupt Flag Status and Clear register (INTFLAG.CFD) is set and the
corresponding interrupt request will be generated if enabled. The BKUPCLK bit in the CTRL register is set by hardware
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_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