Miya Pearson
AVR devices can be put into different sleep modes, which can considerably reduce their power consumption. The simplest way to put an AVR device to sleep is to set the desired sleep mode using set_sleep_mode() and then call sleep_mode. However, there are several other methods and considerations to take into account, such as disabling the Brown Out Detector (BOD) before going to sleep to further reduce power consumption. Additionally, the method for waking an AVR device from sleep depends on the specific mode selected with the set_sleep_mode() function.
AVR Sleep Characteristics and Values
| Characteristics | Values |
|---|---|
| Use | Allows applications to reduce power consumption |
| Sleep Modes | Different modes available, including idle mode |
| Macro | Several macros provided in the header file to put the device into sleep mode |
| Simplest Way to Sleep | Use set_sleep_mode() and then call sleep_mode() |
| Interrupts | Need to be enabled before going to sleep unless the CPU is to be locked |
| Race Conditions | May occur with the sleep_mode() macro |
| Brown Out Detector (BOD) | Can be disabled before sleep to reduce power, but must be done within a limited number of cycles |
| Wake-Up | Depends on the specific mode selected with set_sleep_mode() |
| Sleep Enable Bit | Should be set before sleep and cleared after |
| Sleep Timer | Can be set for the AVR to wake up at specific intervals |
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What You'll Learn
Using the SLEEP instruction to reduce power consumption
AVR devices can be put into different sleep modes to reduce power consumption. The SLEEP instruction is used to put the device into sleep mode, but before that, the Sleep Enable bit in the Sleep Mode Control Register (SMCR.SE) must be set to '1'. The Sleep Mode Select bits (SMCR.SM [2:0]) determine which sleep mode will be activated. The available sleep modes are Idle, Power-down, Power-save, Standby, and Extended Standby.
To reduce power consumption, it is important to disable any functions or modules that are not needed during sleep. For example, the Analog Comparator should be disabled if not required, as it will continue to consume power in Idle mode. Similarly, if the Brown-Out Detector (BOD) is not needed, it should be turned off before entering sleep mode. Disabling the BOD can be done using the sleep_bod_disable() macro, which generates inlined assembly code to implement the timed sequence for disabling the BOD before sleeping.
When entering sleep mode, all port pins should be configured to use minimum power. It is crucial to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will be disabled to prevent unnecessary power consumption. However, if the input logic is needed for detecting wake-up conditions, it should remain enabled.
The specific sleep mode and configuration depend on the AVR device being used. It is recommended to refer to the datasheet for the device to determine the appropriate sleep mode and power management settings. Additionally, it is important to consider the trade-off between power savings and the time required to wake up the device from different sleep modes.
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Different sleep modes for AVR devices
AVR devices can be put into different sleep modes to reduce power consumption. The specific sleep mode to be used depends on the device in question and its unique requirements.
The simplest way to put an AVR device into sleep mode is to set the desired sleep mode using set_sleep_mode() and then call sleep_mode(). This macro automatically sets the sleep enable bit, goes to sleep, and clears the sleep enable bit. It is important to note that unless your purpose is to completely lock the CPU (until a hardware reset), interrupts need to be enabled before going to sleep.
Some devices have the ability to disable the Brown Out Detector (BOD) before going to sleep, which will further reduce power consumption. If the specific AVR device has this ability, the sleep_bod_disable() macro can be used to generate inline assembly code that will correctly implement the timed sequence for disabling the BOD before sleeping.
When entering sleep mode, it is important to ensure that all port pins are configured to use minimum power. This includes disabling digital input buffers for analog input pins, as an analog signal level close to VCC/2 on an input pin can cause significant current draw even in active mode.
There are several sleep modes available for AVR devices, including:
- SLEEP_MODE_IDLE: In this mode, the CPU is put to sleep, but all peripheral clocks are still running.
- SLEEP_MODE_ADC: Details about this mode are not readily available.
- SLEEP_MODE_PWR_SAVE: This mode is identical to Power-down, except that if Timer/Counter2 is enabled, it will keep running during sleep.
- SLEEP_MODE_STANDBY: This mode is identical to Power-Down, except that the Oscillator is kept running. The device wakes up from this mode in six clock cycles.
- SLEEP_MODE_PWR_DOWN: This mode is used to minimize power consumption.
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Disabling the Brown Out Detector (BOD)
To disable BOD in relevant sleep modes, both BODS and BODSE must first be set to one. Then, to set the BODS bit, BODS must be set to one and BODSE must be set to zero within four clock cycles. The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS is active to turn off the BOD for the actual sleep mode.
It is important to note that there is a limited number of cycles after the BOD has been disabled that the device can be put into sleep mode, otherwise, the BOD will not be truly disabled. The recommended practice is to disable the BOD, set the interrupts, and then put the device to sleep.
Some devices do not have the ability to disable BOD before going to sleep. In these cases, the BOD can be disabled globally by fuses, which will result in the same sleep mode power consumption as when BOD is disabled by software.
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Using set_sleep_mode() and sleep_mode() macros
AVR devices can be put into different sleep modes, and there are several macros provided in the header file to put the device into sleep mode. The simplest way is to use the set_sleep_mode() and sleep_mode() macros.
The set_sleep_mode() macro is used to set the desired sleep mode. It can take different parameters depending on the specific AVR device and the available sleep modes. Some common sleep modes include SLEEP_MODE_IDLE, SLEEP_MODE_ADC, SLEEP_MODE_PWR_SAVE, SLEEP_MODE_STANDBY, and SLEEP_MODE_PWR_DOWN. The default mode is usually idle mode, where the CPU is put to sleep, but all peripheral clocks are still running.
The sleep_mode() macro is then called to put the device into sleep mode. This macro automatically sets the sleep enable (SE) bit, goes to sleep, and clears the sleep enable bit. It is important to note that interrupts need to be enabled before going to sleep, unless the intention is to completely lock the CPU until a hardware reset. Additionally, the sleep_mode() macro might cause race conditions in some situations, so it is recommended to manipulate the SE bit and issue the SLEEP instruction manually if necessary.
#include
Void idle() {
Set_sleep_mode(SLEEP_MODE_PWR_DOWN);
Sleep_mode();
}
In this example, the code includes the necessary avr/sleep.h header file and defines a function called idle(). Inside the function, the set_sleep_mode() macro is used to set the sleep mode to SLEEP_MODE_PWR_DOWN. Then, the sleep_mode() macro is called to put the device into sleep mode.
It is important to refer to the datasheet for the specific AVR device to understand the available sleep modes and the recommended practices for using set_sleep_mode() and sleep_mode() macros. Additionally, some devices have the ability to disable the Brown Out Detector (BOD) before going to sleep using the sleep_bod_disable() macro, which can further reduce power consumption.
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Waking up from sleep mode
- External Interrupts: Many microcontrollers, such as the ATmega328P, can be woken up from sleep mode by external interrupts. These interrupts can be triggered by various sources, such as buttons, sensors, or other external events. By enabling and configuring the appropriate interrupts, the microcontroller can be programmed to wake up in response to specific conditions or events.
- Reset: In some cases, a reset may be required to wake up the microcontroller from sleep mode. This can be achieved by pressing a reset button or through software reset mechanisms. However, it is important to note that a reset may also clear the microcontroller's state and require it to restart from a known initial condition.
- Timer Interrupts: AVR microcontrollers often support timer interrupts, which can be used to wake the device from sleep mode after a specified time interval. By configuring timer interrupts and enabling them before entering sleep mode, the microcontroller can automatically wake up at the desired time.
- Interrupt Service Routine (ISR): When an enabled interrupt occurs while the microcontroller is in sleep mode, it wakes up and executes the ISR associated with that interrupt. After handling the interrupt, the microcontroller may need to be put back to sleep explicitly if desired.
- Specific Sleep Modes: Different sleep modes have varying wake-up conditions. For example, the Power-Down mode halts all generated clocks, while the Idle mode allows certain modules to continue operating. Understanding the specific sleep mode and its characteristics is crucial for determining the appropriate wake-up mechanism.
It is important to refer to the documentation and datasheets specific to your microcontroller to understand the supported sleep modes and their respective wake-up methods. Additionally, careful consideration of power consumption, interrupt handling, and timing requirements is essential for successfully implementing sleep and wake-up functionality in AVR microcontrollers.
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Frequently asked questions
The simplest way is to set the desired sleep mode using set_sleep_mode() and then call sleep_mode.
An interrupt will wake the AVR chip. The device will also wake up when a pin state changes, a timer overflows, the watchdog timer times out, or data arrives on a UART.
The SLEEP instruction allows an application to reduce its power consumption considerably.
The sleep_bod_disable() function is used to disable the Brown Out Detector (BOD) before going to sleep, which further reduces power consumption.
To prevent the device from going to sleep, you can remove any sleep function from the ISR and ensure that interrupts are enabled before going to sleep.
