“In many small portable IoT applications, the ultimate challenge for the design engineer is to provide reliable wireless connectivity while operating from a single coin cell battery for ten years.
Article author: Bob Card, Marketing Manager, ON semiconductor
In many small portable IoT applications, the ultimate challenge for the design engineer is to provide reliable wireless connectivity while operating from a single coin cell battery for ten years.
It’s no easy feat, and most cheap coin cells only offer about 240mAh. However, this Design Note demonstrates that by choosing a wireless system-on-a-chip (SoC) with a low sleep mode, it is possible to achieve the decade goal on short- and long-range wireless connectivity.
If an application can only utilize 240mAh of energy, the wireless device must spend most of its time sleeping, waking up only occasionally for wireless transmission (see Figure 1). For example, a wake-up time of 7 ms, a transmission interval of 5s and a duty cycle of 120 wireless transmission actions per hour are 0.14% wake-up time and 99.86% sleep mode. This explains why it is critical that the radio SoC consumes very low power in sleep mode.
Figure 1: Sleep Mode Duty Cycle
For battery powered applications operating over short or medium distance links (usually up to 50m), Bluetooth LE radio technology is preferred. For long-distance transmission over 1000m, ON Semiconductor’s AXM0F243 wireless microcontroller supports Sub 1GHz software-defined radio (SDR) technology.
The Bluetooth LE radio uses the 2.4GHz band and has 40 channels spaced 2MHz apart. Three RF channels (37, 38, and 39) are reserved for advertising functions that allow the discovery of nearby devices. Channels 0-36 are reserved for data transfer. Advertising channels occupy different parts of the spectrum to counteract interference from 802.11 or Wi-Fi radio transmissions (see Figure 2).
Figure 2: Bluetooth LE advertising channel (Source: Accton Marketing)
The data unit of an advertisement packet is called a Protocol Data Unit (PDU) and has a two-byte header that specifies the type and length of the data payload, up to 37 bytes (6 bytes for the advertisement address and 31 bytes for the data) .
Connectable and unconnectable transports
Bluetooth LE advertising packets can be “connectable” or “unconnectable”. Figure 3 shows the operation of ON Semiconductor’s RSL10 System-on-Chip (SIP) wireless SoC as measured by a power analyzer and shows connectable (left) and unconnectable (right) advertising events, both at 0dbm transmit power.
While both events use channels 37, 38, and 39 and last for 7 milliseconds, the connectable events include receive pulses for each channel. This makes sense since connectable events are also designed to receive incoming transports. The resulting power analyzer measurements revealed the average current for each transfer type: 711.624µA for connectable events and 504.307µA for unconnectable events.
Additionally, the RSL10 SIP has a deep sleep current of 160nA, reserves 16kbytes of data in RAM for the Bluetooth LE stack, and runs an internal timer to wake itself up.
Figure 3: Power consumption to transmit connectable and unconnectable packets
RSL10 SIP Battery Life
Figure 4 demonstrates that, under the above conditions, the actual battery life of an RSL10 SIP-based system ranges from 10.97 years for connectable transmissions at 2.5s advertising intervals to 27.26 years for non-connectable transmissions at 5s advertising intervals. These calculations assume a 240mA CR2032 coin cell battery and a 5-byte PDU.
Figure 4: Actual battery life using RSL10 SIP application
For longer-distance wireless transmission, ON Semiconductor’s AXM0F243 narrowband SoC achieves a link budget of 153dB and can transmit a distance of 37 kilometers (23 miles) while operating with 30dB attenuation at 915MHz. For transmission within 1.1 km, the AXM0F243 can exceed the required 10-year battery life (see Figure 5).
Figure 5: Actual battery life for remote applications based on AXM0F243 SoC
So with the right radio SoC, it’s entirely possible to achieve 10 years of battery life at short or long range.