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Power Consumption

  • Can ANT be used with energy harvesting?
    • Yes, ANT was designed as an ultra low power wireless solution and is ideal for use with energy harvesting. 

  • I am using the SLEEP line, but it seems ANT only goes into its lowest power mode when the channel is closed. Why is that?
    • Please note that “Sleep” mode is not the lowest power state of ANT. 

      “Sleep” mode takes place in between serial transmissions, i.e., the SLEEP line is used to indicate to ANT that the host will not be using the serial port.  By using the SLEEP line you will avoid consuming power for the serial port when you do not need to send serial messages, but RF activity is still possible. While SLEEP is asserted, ANT will be switching back and forth between “Sleep” and “Active” states as there is RF activity.

      The lowest power state is “Idle” state (unless “Deep Sleep” is available).  To go into “Idle” mode, make sure to close all ANT channels, and make sure that the SLEEP line is asserted.

      Please refer to application note AN13 Power States for additional information on ANT power states.

  • What are the maximum theoretical search times for a given message period?
    • The worst case acquisition time for common transmission message periods is shown below:

      Transmission Worst case search time
      10 Hz 2 seconds
      4 Hz 3 seconds
      2 Hz 7 seconds
      1 Hz 15 seconds
      0.5 Hz 45 seconds

      There is no mathematical formula for calculating these numbers. These estimates were determined using a brute force method. These numbers also assume zero packet loss and a favourable RF environment.

  • How do I select an optimal message period for my application?
    • ANT configuration is very flexible, and different parameters of the protocol can be varied between the master and slave nodes to meet the power and latency requirements of your application.  In order to select the best message period for a specific application, you need to consider your requirements in terms of latency, power, how much data needs to be transmitted, how often, and what type of data.

  • Is it necessary to use the SLEEP line while using synchronous communication?
    • No. Power management of the serial port is done automatically by ANT when using synchronous communication.

  • What is the relationship between message period and power consumption?
    • The power consumption increases with increasing message rate (i.e decreased message period), assuming that the channel is synchronized. During channel acquisition, search time increases with slower message periods, therefore power consumption increases at the slave.

      Search time depends on the message period of the master.  To reduce power consumption at the slave, without affecting search time, it is possible to decimate the message rate of the slave relative to the master. This is only possible if the application is tolerant to missed data packets and increased latency. In order to maintain synchronization, the message rate of the slave should be a factor of the rate of the master, for example, if the master is configured at 8Hz, the receiver could be configured at 2, 4 or 8 Hz.

  • What is a good method of estimating the power consumption of my link?
    • The online Power Estimator is a useful tool for estimating the current consumption for different ANT products, serial modes, channel types, message periods, data types and use cases.

      For example, consider a setup with an AP1 in byte synchronous serial mode, using EXT32K.   If we want to know the current consumption with the device configured as a master, transmitting broadcast data at 8Hz, one hour per day, we could use the online Power Estimator as shown in the image below.

      Our data sheets also contain detailed current consumption figures that will allow you to calculate the lifetime for more complex use cases not considered in the Power Estimator.

      For the example above, using the datasheets, we can obtain the total average current consumption as follows:

      I_ave = I_Base_EXT32K + 8Hz * I_Msg_TR_ByteSync = 35uA + 8Hz * 26uA/msg = 243uA

      As another example, consider the same device, configured as a master at 0.5Hz, and transmitting a burst of 80kbyte of data @ 20kpbs every 15 minutes. 

      At 20kbps, each burst transmission takes 80kbytes * 8/20kbps = 32 seconds.

      The average current consumption of the burst over a 15 minutes interval is

      I_ave_burst = (4.9mA * 32 s) / (15 * 60 s) = 174.22uA

      And the total average current is

      I_ave = I_Base_EXT32K + 0.5Hz * I_Msg_TR_ByteSync + I_ave_burst = 35uA + 0.5Hz * 26uA/msg + 174.22uA = 222.22uA.

  • Are there some guidelines for minimizing power consumption at the hardware level?
    • For most ANT solutions, use of an external 32 KHz clock source will result in a lower base current. Refer to the datasheets for specific current consumption figures.

      When using asynchronous serial communication with ANT (UART), proper use of the SLEEP line will allow the serial port to be shut down when not in use.

  • If ANT enters into “Sleep” mode, how can the host know when ANT should be woken up?
    • ANT will automatically switch between “Sleep” and “Active” modes as there is RF activity. The host can also use the SLEEP signal to indicate to ANT that serial communication from the host to ANT is required.

  • My application is very power sensitive, and does not have very strict latency requirements. Is it possible to transmit slower than 0.5 Hz to reduce power consumption?
    • The slowest channel period supported by ANT is 0.5 Hz, however, it is possible to use an intermittent transmission scheme, keeping the channel open only when required.  Note that this increases the power consumption at the slave, since it must search for the master device in every transmit interval.  Depending on your latency requirements, it may be more power efficient to keep the channel open and synchronized than using intermittent transmission.

      If using a receiver in continuous scanning mode, there is no search involved, so it is possible for the master device to send data only when required by your application.

  • Current consumption on my custom design is way off the figures shown in the Power Estimator. What could be causing this?
    • Make sure that your measurements take into consideration the power consumed by the rest of your system.  Note that the Power Estimator lets you select the type of serial communication and whether or not you are using an external 32 KHz clock source. Please make sure that the settings of the calculator match your hardware setup.  The calculations in the Power Estimator for UART assume proper use of the SLEEP line; if you are not currently implementing the SLEEP line, you will see a difference in the resulting current consumption.

      If your figures are still considerably off, the problem could be caused by incorrect hardware connections or a damaged chip. Please check the schematics and hardware circuit, especially the pin out around the serial lines, to ensure there are no shorts. If the problem still exists, please contact Nordic Semiconductor or Texas Instruments for hardware support.

  • How do I decide which device should be the slave and master?
    • The decision on which device should be the slave and master is application specific, and depends on the desired topology, power constraints and even user interface considerations on both sides of the link.

      Channels are bidirectional, so the master does not need to be the data source. The slave will incur higher power consumption during channel acquisition, so generally it is desirable to set the most power constrained device as the master.  In some cases, the topology will restrict this decision (e.g. in a shared channel the hub is always the master).

      For extremely power constrained devices, it is possible to further offload the power consumption to the receiver by using the continuous scanning mode.