Data Sheet

162
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
16.6.9.3 Sampling Mode
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.
Figure 16-7 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.
Figure 16-7. Sampling Mode Block diagram
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.
16.6.9.4 Hysteresis
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.
16.6.10 Voltage Reference System Operation
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
F
clksampling
F
clkprescaler
2
PSEL 1+()
------------------------------
=
US
ER INTERFA
CE
R
E
G
I
S
TER
S
(
APB clock domain
)
P
RE
SC
ALE
R
(
clk_prescale
r
domain
)
S
YN
C
HR
O
NIZER
P
S
EL
C
EN
MO
DE
ENABL
E
C
LK_APB
C
LK_PRE
SC
ALER
C
LK
_
SAMPLIN
G