In the process of reducing design power consumption, have you taken full advantage of all the functions of the integrated analog-to-digital converter (ADC) in the microcontroller (MCU)? This blog post will show you how to achieve lower power consumption with an integrated analog.
In this article, we will use ADC14 (integrated 14-bit analog-to-digital converter) in the MSP432P401R MCU as an example. Low-power applications, as well as reduced startup time in high duty cycle applications, are all factors in the design of the ADC14. However, each application has its own unique characteristics, so to minimize power consumption, you must carefully select the knob or programmability of the ADC14.
This blog post focuses on some of the key features of the MSP432TM MCU that allow you to customize the power and performance of the ADC14:
Optional Reference Quick Start Optional Clock Source Power Mode Minimum Voltage 1.62V Use integrated DC / DC drive core voltage to automatically shut down the internal temperature sensor, ADC sampling time is reduced by 8, 10, 12 or 14 bits, select the lowest number of digits to increase Conversion speed, saving battery power window Comparator does not have to do the actual processing before discovering the relevant signal, even without using the 8-bit mode DMA module process to use the timer to trigger the ADC conversionOptional reference
The optional reference allows the user to select the minimum current that is appropriate for their performance. If the power supply is stable, the power supply can be used as an ultra low power reference. Using a power supply as a reference means that the internal reference does not require current and the reference has no startup time.
Quick Start
The ADC14 starts up quickly, further improving the low power consumption of high duty cycle applications. The startup time of the ADC and clock (MODOSC or SYSOSC) is very short. In addition, before the buffer is started (see the device data sheet for specific values), it is first started as an internal reference for low power consumption. The buffer can be set up quickly because there is no need for an external capacitor that takes too long to charge. In this way, the buffer needs to be turned on only when it is used, and it takes longer for the external capacitor to charge.
Optional clock source
When considering clock selection, you need to consider the system level power budget. In some cases, converting a faster clock saves energy. Work cycle applications can benefit from MODCLK with fast startup time. Users must consider increasing the current of different clock sources to minimize ADC startup time and save net power.
Power mode
The power mode (ADC14PWRMD bit) adjusts the current consumption at the maximum sample rate, primarily by adjusting the buffer used when selecting the internal reference. As in the case of SYCOSC, if you are using a slower clock in ADC14, consider using the low power mode (ADC14PWRMD = 2) as the clock source (see the device data sheet for specific clock requirements).
When using an external reference, the energy differential voltage for each transition between ADC14PWRMD settings is as small as when the reference buffer is not being used. In this case, a slower clock reduces the current consumption of the ADC, but it takes longer to complete.
When using internal references, the minimum power mode depends on your application. The following factors should be considered based on each application, including: energy saving when the ADC is passive, enabling sampling time, number of conversions, reference used for clock or other places, clock frequency, number of conversions, and so on. For applications with long sampling times, the ADC sample time current is less than the conversion current, so the data you see is smaller than the value in the data table. You may need to do some bench testing to understand the ADC current consumption of your application.
Using an internal reference with minimal sample time and considering the energy of MODOSC / SYSOSC, the low power mode of a single ADC conversion consumes the least amount of energy. However, with five or more consecutive conversions, the conversion speed begins to dominate, while the conventional power mode with a faster clock consumes the least. Figure 1 compares the power consumption of two different conversion times in the 12-bit mode.
Figure 1 ADC with internal reference
To help you optimize your system, Figure 2 shows the current distribution for two examples of ADC14 with internal reference in normal and low power modes.
Figure 2 ADC with internal reference library
During full-speed operation, the ADC14 supports an optimum minimum supply voltage of 1.62 volts when ADC14PWRMD = 2 (200ksps maximum) or a minimum supply voltage of 1.8V. For battery operation, if low power mode is available, battery life is extended and the signal is still fully sampled. For regulated power supplies, using a low-voltage buck converter can greatly increase the efficiency of all current sources and reduce the current drawn from the power supply.
Ability to use integrated DC / DC drive core voltage
The MSP432 microcontroller provides an integrated DC/DC converter that improves core power efficiency including ADC14 digital logic. For the number field of the ADC14 current, the DC/DC converter reduces current consumption. For differential inputs, the performance difference is negligible when using a DC/DC converter. For single-ended input, this has a small effect on 70 dB versus 73 dB typical SINAD (signal-to-noise ratio and distortion ratio). For more information, please refer to the device data sheet to ensure that the ADC14 and DC/DC converters are suitable for your application.
Automatic shut-down
The ADC14 features an automatic shutdown feature that reduces power consumption without any user intervention. When ADC14 stops the conversion operation, the processor is automatically disabled and automatically re-enabled when needed. The clock source, MODOSC or SYSOSC can also be automatically enabled to automatically provide MODCLK or SYSCLK to ADC14 when needed; it can also be disabled if ADC14 or the rest of the device is not needed. ADC14 MODOSC / SYS OSC is started with the internal reference, so the clock auto-shutdown has no effect.
The internal reference does not automatically power down between the sample or conversion phase by setting the ADC14REFBURST bit and setting the REFON bit to '0'.
Internal temperature sensor
The internal temperature sensor design has a shorter sampling time requirement than previous MSP devices to reduce the energy used to measure temperature.
The last four precautions are described in more detail in the second and third blog posts of this series:
Choosing the minimum number of digits required increases the speed of completion and saves energy.
With the window comparator, you don't have to actually process the comparison conversion value before you complete the match and increase the resolution, even without using the 8-bit mode.
Use DMA module process to reduce resource usage
Use a timer to trigger ADC conversion to minimize the resources used
How many of these features can you use to reduce the ADC power consumption of your MSP432 MCU in your application?
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