To achieve low power consumption as a wireless microcontroller module for connecting things, it is necessary to suppress the current consumption of each process, but it is also very important to shorten the startup time and processing time. Even if the current consumption is low, low power consumption cannot be achieved if the processing time is long.
Power Consumption (Current)
The following are typical current consumptions when sending one packet of 30 bytes.
Standard output BLUE
In the case of TWELITE BLUE
High output RED
In the case of TWELITE RED
High output RED has a higher current consumption during transmission compared to standard output BLUE, but since the processing time is shorter, the current consumption for sending one packet is equivalent.
TWELITE achieves power saving through low current consumption, instant startup, and high-speed processing.
The current consumption during reception is BLUE 17.0mA and RED 14.7mA. If continuous reception is required, constant power supply or a battery with large capacity is necessary.
Please also consider the power consumption of peripheral circuits such as sensors during development.
The pre-installed Extremely Simple! Standard App at shipment is not designed for power saving. If you want to pursue power saving, please modify the app.
Examples of Power Consumption
Here is an example of a wireless tag with minimized power consumption of TWELITE. In this example, no sensors are used, and 4 bytes of ID data are periodically sent using the built-in timer. No reception acknowledgment (ACK) from the transmission counterpart is performed. The average current consumption when sending one packet is 11.6mA, and the processing time is 2.5ms.
Average current consumption considering only the awake time
11.6[mA] * (2.5/1000)[ms] = 29[uA]
Average current consumption including sleep (RAM on) time
29[uA] + 1.5[uA] = 30.5[uA] = 0.0305[mA]
Assuming transmission once every second and a CR2032 battery capacity of 220mAh,
The graph below shows the battery life when using a coin-type battery (CR2032). The battery life exceeds 10 years when data is sent every 30 seconds.
If sending intervals are less than 5 seconds, it is better to keep RAM on and sleep to extend battery life.
The values obtained by this calculation are approximate and may vary depending on battery performance and operating temperature.
Due to the current supply capability of coin batteries (please refer to the battery manufacturer’s datasheet for details), continuous operation of the wireless module is not possible. When using coin batteries, please perform intermittent operation including sleep. More information
The battery life introduced here is a reference value and may be longer or shorter in reality. Battery life varies depending on its performance and operating conditions. (Pulse load [generally extends life], instantaneous overload current [reduces life], temperature conditions [life decreases at high temperature, voltage drops at low temperature]) Please check the characteristics of the battery you are using in detail. Please be especially careful about voltage drops in batteries with high internal resistance.
1 - Battery Operation and Buffer Capacitor
A buffer capacitor is used when operating TWELITE with a small-capacity battery
When using a small-capacity battery, it may not be able to supply sufficient current. For example, the rated current of a CR2032 is about 0.2mA, and even at maximum, only around 2~3mA is expected.
For instance, when TWELITE transmits a wireless packet, it momentarily consumes about 20mA. At this time, insufficient current supply and voltage drop may occur, causing the system to malfunction.
To avoid this, a buffer capacitor is connected between VCC and GND. (Additionally, it is necessary to control startup using a reset IC.)
Assuming the power is cut off, with a voltage of 3.3V and an average current of 10mA, a 100uF buffer capacitor can keep the system running for about 10ms. From the moment the power is cut off, current is supplied from the buffer capacitor, and the wireless module will operate until the voltage drops near 2V, where the wireless module becomes inoperable.
This is an effective measure for intermittent operation and transmission. A caution is that if retransmissions occur, the time until transmission completion significantly increases. Since one retransmission consumes several milliseconds, a buffer capacitor considering the number of retransmissions is necessary.
However, in cases where the device waits for reception, it continuously consumes a large current (17.2mA), so dealing with this using the above buffer method is difficult. Please use a battery with a higher supply capacity.
Approximate Capacitor Capacity and Operating Time
The current consumption of TWELITE series modules does not depend on the supply voltage. Because the current is constant, we calculate how long the module can operate based on the capacitor’s charge.
If a capacitor with capacitance C stores charge corresponding to voltage v, the time t for the voltage to drop from v to 0 at a constant current i is t = Cv/i.
The time required for the voltage to drop from 3.3V to 2.1V at current i is C(3.3-2.1)/i. For i=10mA and C=100uF, this calculates to 12ms. According to this calculation, if the power to a module operating at 3.3V is suddenly cut, and the average current is 10mA, the operation time can be extended by about 12ms.
The current value of 10mA is only an assumed average. Please refer to actual measured consumption current values.
