Atmel Xplained Pro Evaluation Kit for the ATSAMD21J18A Microcontroller ATSAMD21-XPRO ATSAMD21-XPRO Data Sheet

152

Atmel | SMART SAM D21 [DATASHEET]

Atmel-42181C–SAM-D21_Datasheet–07/2014

The sampling mode is a low-power mode where the BOD33 is being repeatedly enabled on a sampling clock’s ticks. The 
BOD33 will monitor the supply voltage for a short period of time and then go to a low-power disabled state until the next 
sampling clock tick.

Sampling mode is enabled by writing one to BOD33.MODE. The frequency of the clock ticks (F

clksampling

) is controlled by 

the BOD33 Prescaler Select bit group (BOD33.PSEL) in the BOD33 register.

The prescaler signal (F

clkprescaler

) is a 1kHz clock, output from the32kHz Ultra Low Power Oscillator, OSCULP32K. 

As the sampling mode clock is different from the APB clock domain, synchronization among the clocks is necessary. 

 shows a block diagram of the sampling mode. The BOD33Synchronization Ready bits (PCLKSR.B33SRDY) 

in the Power and Clocks Status register show the synchronization ready status of the synchronizer. Writing attempts to 
the BOD33 register are ignored while PCLKSR.B33SRDY is zero.

The BOD33 Clock Enable bit (BOD33.CEN) in the BOD33 register should always be disabled before changing the 
prescaler value. To change the prescaler value for the BOD33 during sampling mode, the following steps need to be 
taken:

1. Wait until the PCLKSR.B33SRDY bit is set.

2. Write the selected value to the BOD33.PSEL bit group.

The hysteresis functionality can be used in both continuous and sampling mode. Writing a one to the BOD33 Hysteresis 
bit (BOD33.HYST) in the BOD33 register will add hysteresis to the BOD33 threshold level.

The Voltage Reference System (VREF) consists of a Bandgap Reference Voltage Generator and a temperature sensor.

The Bandgap Reference Voltage Generator is factory-calibrated under typical voltage and temperature conditions.

At reset, the VREF.CAL register value is loaded from Flash Factory Calibration.

The temperature sensor can be used to get an absolute temperature in the temperature range of CMIN to CMAX 
degrees Celsius. The sensor will output a linear voltage proportional to the temperature. The output voltage and 

2

PSEL 1

+

(

)

------------------------------

=

USER INTERFACE

REGISTERS

(APB clock domain)

PRESCALER

(clk_prescaler

 domain)

SYNCHRONIZER

PSEL

CEN

MODE

ENABLE

CLK_APB

CLK_PRESCALER

CLK_SAMPLING