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ON CURRENT OFF CURRENT DETAILS
Panasonic AMN41121 PIR sensor
presence sensor
60 µA ~ 0 nA Digital output stabilizes in: 7 s - 30 s
Silicon Labs Si7034
humidity sensor
245 µA ~ 50 nA I2C interface sample time: 3.7 ms - 6 ms
muRata NTC thermistor
temperature sensor
33 µA ~ 0 nA -

Table 1 - Example component properties. Note that values will vary greatly depending on the chosen components.

 
HIGH-FREQUENCY PERIPHERALS
Peripherals requiring a clock in the MHz range. Includes interfaces like UART and USB, high frequency timers, DMA, accelerator engines, etc.
 
LOW-FREQUENCY PERIPHERALS
Peripherals running off of a slower clock, often 32 kHz, to conserve energy. These allow a high level of functionality even in deep sleep modes. Includes communication interfaces like the UART, sensor interfaces like LESENSE, etc.
 
ASYNCHRONOUS PERIPHERALS
Using no clocks, these peripherals typically respond to externally generated events. Examples are the pulse counter (PCNT), and the I2C when in slave-mode.
 
IO STATE AND WAKEUP
The ability to retain the state of the MCU pins and also waking up, giving control back to software, are key even in the lowest energy modes.
 
 
ENERGY MODE ASSOCIATED NAME EXAMPLE BASE
CURRENT CONSUMPTION
CPU HIGH-FREQUENCY
PERIPHERALS
LOW-FREQUENCY
PERIPHERALS
ASYNCHRONOUS
PERIPHERALS
I0 STATE AND WAKEUP
EM0 Run 114 µA/MHz Y Y Y Y Y
EM1 Sleep 48 µA/MHz - Y Y Y Y
EM2 Deep sleep 0.9 µA - (some) Y Y Y
EM3 Stop 0.5 µA - (some) (some) Y Y
EM4 Shutoff 20 nA - - (some) (some) Y

Table 2 - Overview of energy modes on EFM32 Gecko MCUs.

Sleep Mode
A system is said to be sleeping when its main coordinating function is powered down. For a microcontroller, sleep would mean that the CPU has stopped executing code. Since executing code consumes energy, sleeping conserves energy. With deeper levels of sleep, larger parts of the system is sleeping, giving higher energy savings, but with deeper sleep also comes the downside of less functionality available and longer wakeup times.
The EFM32 MCUs are designed to maximize the amount of time that can be spent in sleep modes, also known as energy modes. This is achieved by providing a broad amount of functionality in sleep modes, combined with fast wakeup times.

RESPONSE TIME

Response time is the length of time taken by a system to react to a given stimulus or event. Faster response times often come at the expense of power consumption, because the event has to be checked for more frequently, and because once the event has been detected, the system needs to be able to respond in time, which could involve waking up from sleep, and the deeper sleep modes require longer wakeup times.

LESENSE

LESENSE is a peripheral available on some EFM32 devices, which allows autonomous monitoring of external sensors for things like temperature, capacitive touch, presence of metal, and many other things. LESENSE can monitor up to 16 sensors, and can also autonomously combine sensor results and make decisions based on these without waking the EFM32 up from sleep. The CPU is only woken up whenever LESENSE deems it necessary. The ability to stay in deep sleep modes for the majority of an application’s lifetime has a significant, positive impact on energy efficiency because sleep modes require longer wakeup times.
PRS CH INPUT AND OR INV RESULTING OUTPUT
0 ADC ADC conversion done
1 PRS_CH3 PRS_CH0 Y !(CH0 II CH1)
2 ADC Y !ADC conversion done
3 PRS_CH1 PRS_CH2 Y !(CH2 II CH3)
4 PRS_CH3 PRS_CH5 CH4 && CH5
Enable sensor
5 RTCC RTCC event
Start conversion

Table 4 - PRS configuration to implement circuit performing optimal excitation of external sensor.

CPU MOST EFFICIENT ON
Cortex M4 Large applications and/or applications with signal processing
Cortex M3 Large applications with mixed requirements
Cortex M0+ Smaller applications, stacks, control logic

Table 5 - Simplified overview of ARM Cortex M CPUs