ST AN1605 Application note

AN1605

APPLICATION NOTE

USING AN ACTIVE RC TO WAKEUP THE ST7LITE0 FROM POWER SAVING MODE

by Microcontroller Division Applications

1 INTRODUCTION

This application note investigates the power consumption during the operation of a typical application which puts the MCU (ST7Lite0) in HALT mode and wakes it up at regular intervals by an external interrupt generated by an active RC circuit.

It also lists the typical consumption values and the parameters on which this consumption depends. The internal Pull up (Rpu) of an I/O port is used for the active RC to minimise the number of external components. The value of the Rpu varies with the supply voltage of the MCU and with temperature. Externally, only one capacitor is used.

The ST7LITE0 clock source is configured by option byte to be internal RC with PLL*8. All measurements are taken at ambient temperature.

1.1 HARDWARE SOLUTION

Figure 1 shows the Hardware setup used to measure the consumption (IDD) at different power supply voltages.

Figure 1. Hardware Setup

VDD

PA0 EI0 RPU

CEXT

1.2 POWER CONSUMPTION

The power consumption depends mainly on the time period between each wakeup from HALT (external interrupt interval). The wakeup time period is controlled by the RC time constant. The value of the Cext is fixed but the internal pull-up value varies with the MCU power supply voltage and with temperature.

AN1605/0403

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The typical values of the Rpu at different power supply voltages is given in Table 1.

Table 1. Rpu values for different power supply conditions

Setup No.	Conditions	Rpu (Minimum)	Rpu (Typical)	Rpu (Maximum)
1	VDD = 5.0 V	50 kΩ	120 kΩ	250 kΩ
2	VDD = 3.0 V	-	160 kΩ	-

The consumption is measured for two different values of Cext, given in Table 2 and Table 3.

Table 2. Consumption for Cext = 1 µF

			Average IDD (mA)	Average IDD (µA)		IDD (µA)
				Consumption in	Ext Interrupt	Consumption
	Setup No.	VDD (Volts)	Consumption in
				Power saving	Interval (ms)	when MCU is in
			Run mode
				mode		HALT
	1	2.4	1.44	15.6	154.0
	2	2.6	1.73	20.6	131.2
	3	2.8	2.0	26.6	114.0
	4	3.0	2.28	35.2	101.2
	5	3.2	2.59	41.8	90.8
	6	3.4	2.95	48.6	80.8
	7	3.6	3.34	56.6	72.4	0.1
	8	3.8	3.55	66.2	67.2
	9	4.0	3.73	75.3	62.6
	10	4.2	3.93	83.4	59.2
	11	4.4	4.12	95.4	55.2
	12	4.6	4.31	105.2	53.0
	13	4.8	4.54	117.4	50.6
	14	5.0	4.75	129.4	48.0

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Table 3. Consumption for Cext = 0.47 µF

			Average IDD	Average IDD (µA)		IDD (µA)
	Setup No.	VDD (Volts)	(mA)	Consumption in	Ext Interrupt	Consumption
			Consumption	Power saving	Interval (ms)	when MCU is in
			in Run mode	mode		HALT
	1	2.4	1.45	19.6	69.8
	2	2.6	1.74	25.1	60.0
	3	2.8	2.01	32.1	52.2
	4	3.0	2.29	39.0	46.6
	5	3.2	2.60	46.2	42.1
	6	3.4	2.95	54.2	38.0
	7	3.6	3.34	62.5	35.2	0.1
	8	3.8	3.54	71.7	32.8
	9	4.0	3.75	81.2	30.8
	10	4.2	3.91	91.8	29.2
	11	4.4	4.12	101.9	27.7
	12	4.6	4.31	112.6	26.5
	13	4.8	4.51	124.0	25.4
	14	5.0	4.73	135.4	24.3

The parameters on which the consumption depends are as follows:

■The MCU inserts a delay of 256 CPU cycles to stabilize the internal RC, during this time it consumes some current.

For example at 3.6V, the details of the duration of different modes are as follows:

–MCU in Halt mode: 72.00 ms

–MCU in Run mode (delay period + capacitor discharge time) : 211.2 µs

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