How to correctly measure the power consumption of wireless communication modules in the era of low power consumption?

Low power consumption is an extremely important concept in the Internet of Everything. Most of the Internet of Things nodes need to be powered by batteries. Only by correctly measuring the power consumption of the wireless module can we accurately estimate how much battery will be used for 5 years of battery life. This article will explain the detailed measurement method for you.
In many applications of the Internet of Things, the terminal equipment is usually battery-powered, and the available power is limited. Due to the self-discharge of the battery, the actual power used in poor conditions is only about 70% of the nominal power. For example, the commonly used CR2032 button battery has a nominal capacity of 200mAh, but only 140mAh may actually be usable.
Since the power of the battery is so limited, it is very important to reduce the power consumption of the product! Let's first understand the commonly used methods of measuring power consumption. Only when these methods of measuring power consumption are clear can we optimize product power consumption.
One, power consumption measurement
The power consumption test of the wireless module mainly measures the electric current, here is divided into two kinds of different tests of static electric current and dynamic electric current. When the module is in the dormant or standby state, since the current does not change and maintains a static value, we call it a quiescent current. At this time, we can use a traditional multimeter to measure, just connect a multimeter in series with the power pin to get the value that needs to be measured, as shown in Figure 1.


Figure 1 Multimeter test
When measuring the emission current in the normal working mode of the module, since the time required for signal emission is very short, the entire current is in a state of change, which we call dynamic current. The response time of the multimeter is relatively slow, and it is difficult to capture the changing current, so the multimeter cannot be used to measure the changing current. For the changing current, an oscilloscope and current probe are required to measure. The measurement result is shown in Figure 2.


   　Figure 2 Current probe measurement result graph
2. Calculation of battery life
Wireless modules often have two working modes, working mode and sleep mode, as shown in Figure 3 below.


Figure 3 Average current
The above data is from our LM400TU product. According to the figure above, the sending interval between two sending packets is 1000ms. Calculate the average current:


That is to say, the average current in 1 second is about 2.4mA. If you use a CR2032 to supply power, it can be used for about 83 hours, about 3.5 days under ideal circumstances. What if we extend the working hours to 1 hour? Similarly, it can be calculated by the above formula that the average current in 1 hour is only 1.67uA. The same CR2032 battery can support the device to work for 119,760 hours, about 13 years! It can be seen from the comparison of the above two examples that increasing the time interval between sending packets and prolonging the sleep time can reduce the power consumption of the whole machine and enable the device to work longer. This is why products in the wireless meter reading industry generally have a long service life, because they only send data every day.
Three, common power consumption problems and causes
In order to ensure the low power consumption of the product, in addition to increasing the packet interval time, there is also to reduce the current consumption of the product itself, which is the Iwork and ISleep mentioned above. Under normal circumstances, these two values should be consistent with the chip data manual, but if the user uses it improperly, problems may occur. When we tested the emission current of the module, we found that whether the antenna is installed has a great influence on the test result. When measuring with an antenna, the current of a product is 120mA, but if the antenna is unscrewed, the test current soars to nearly 150mA. The abnormal power consumption in this case is mainly caused by the mismatch of the RF terminal of the module, which causes the internal PA to work abnormally. Therefore, we recommend that customers be sure to carry the load test when evaluating the wireless module.
In the previous calculation, when the sending interval becomes longer and longer, the duty cycle of the working current becomes smaller and smaller. At this time, the factor that affects the power consumption of the whole machine is ISleep. The smaller the ISleep, the longer the product life. This value is generally close to the chip data sheet, but we often encounter high sleep currents reported by customers in the test. Why is that?
This problem is often caused by the configuration of the MCU, and the power consumption of a single IO port of a general MCU can reach the mA level. In other words, if the status of an IO port is accidentally missed or mismatched, it is likely to destroy the early low-power design. Let's take a product as an example to conduct a small experiment to see how big the impact of this problem is.


Figure 4 Low-power IO configuration test results of product A


Figure 5 Test result of improper IO configuration of product A


       In the test process of Fig. 4 and Fig. 5, the test object is the same product, and the module is also configured in the sleep mode. You can clearly see the difference in test results. In Figure 4, all IOs are configured as input pull-down or pull-up, and the tested current is only 4.9uA. In Figure 5, only two of the IOs are configured as floating inputs, and the test result is 86.1uA.
If the working current and duration of Figure 3 are kept unchanged, the sending interval is 1 hour, and different sleep current calculations are brought in. According to the result of Figure 4, the average current of one hour is 5.57uA, and according to Figure 5, it is 86.77uA, a difference of about 16 times. It also uses a 200mAh CR2032 battery to supply power. The product can work normally for about 4 years according to the configuration in Figure 4, and the result is only about 3 months according to the configuration in Figure 5!
It can be seen from the above example that the following design principles should be followed to extend the use time of the wireless module as much as possible:
1. Under the condition of meeting customer application requirements, extend the packet sending interval as much as possible to reduce the working current in the working cycle;
2. The IO status of the MCU must be configured correctly. MCUs of different manufacturers may have different configurations. Please refer to the detailed information.