Continuously reducing the power consumption of integrated circuits is a constant topic in the development of integrated circuits. Reduced power consumption helps save power, extend standby time and reduce product heat, etc. Needless to say, low power consumption has become one of the important factors to measure product performance.
We usually recommend customers to refer MCUXpresso SDK power_mode_switch_lpc demo as low power design reference code. With this demo, customers can use serial terminal to control MCU to enter four low power consumption modes：Sleep mode，Deep Sleep mode，Power Down mode and deep power down mode. Meanwhile, user can also choose a variety of wake-up methods to wake up MCU through UART command.
However, when customers take use of power_mode_switch_lpc demo to measure lower power static characteristics, they find discrepancies with data sheet. Take example with LPCXpresso845MAX board. In power down mode, the demo board current reaches to around 100uA in debug mode. But data sheet states the typical current value is 1.5uA, no more than 10uA (see the Table below). Where is the problem?
The purpose of the power_mode_switch_lpc demo is to demonstrate several low power modes and multiple wake-up methods to customers. We can’t get similar low power current value with default demo board as spec shows, but let's walk through a step-by-step demonstration and modify the routine to get the data sheet values.
LAB ENVIRONMENT:
Demo Board: LPCXpresso845MAX
SDK: SDK_2.11.0_LPCXpresso845MAX
Demo Code: power_mode_switch_lpc
IDE：MCUXpresso IDE v11.5.0
STEP:
1. Download power_mode_switch_lpc to LPC845 development board and run it. The serial port selects low power mode, press (SW2 button) to wake up. The program runs into power down mode, with debugger connected, the measured Idd is 99.5uA
To enter low power mode, the following code is used:
POWER_EnterPowerDown(DEMO_ACTIVE_IN_DEEPSLEEP);
In order to wake up, parameter DEMO_ACTIVE_IN_DEEPSLEEP is configured with PDSLEEPCFG. BOD and watchdog oscillator power domains are turned on. All these setting results in potential current loss, causing power supply current higher than expectation.
2. In this step, we will remove wakeup initialization code
// DEMO_InitWkt();
Replace this line of code
//POWER_EnterPowerDown(DEMO_ACTIVE_IN_DEEPSLEEP); //enter power down mode with BOD and watchdog osc
with
POWER_EnterPowerDown(0); //power down BOD and watchdog osc
The modification is to turn off BOD power domain and watchdog clock in power down mode, compile and download the code again and enter the power down mode. At this time, the measured Idd is 57.3uA
In this way, Idd is significantly reduced. However, 57.3uA is still far from the typical value of 1.5uA stated in the data sheet. This is due to MCU debug power domain is turned on by IDE debugger, which leads to extra current consumption.
3. Thus we disconnect debugger and let the development board work in stand-alone mode (power off and re-power on). After power on, LPC is in power down mode. At this time, the measured Idd is 1.4uA. This is quite similar as the current value in datasheet.
Consider GPIO configuration:
The spec data shown in datasheet are tested under a dedicated test board with almost no external peripheral devices, and unused pins are basically in a floating state. For custom board or LPCXpresso804 EVK. Some of its IO pins have external pull-up resistors and LEDs. If the GPIO is configured to output low power, the small light will be lit, resulting in additional power consumption. So we configure all pins as pull-up inputs for low power measurement.
Consider package:
The test is based on LPCXpresso845MAX equipped LPC845 64pin package. It also applies to LPC845 48pin package.
33 package doesn't have VDDA pin. Due to VDD and VDDA pin design are not exactly the same as other package, for instance, 33pin low power consumption in power down mode is slightly higher than 64/48pin, but no more than 10uA as our spec in DS.
Summary:
Low power current parameters of the data sheet are measured with all MCU oscillators and analog domains off. Besides, reasonable configuration of GPIO can further reduce MCU power consumption. Before entering the low power consumption mode, it is recommended to set the unused GPIO as a pull-up input according to the actual situation (It can also be set to output low when the pin is floating). In applications with high requirements for low power consumption, users need to carefully optimize the code design to obtain the best low power consumption design.
