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 exter nal com ponents. The value of the Rpu varie s with the supp ly voltag e 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 (I
supply voltages.
Figure 1. Hardware Setup
V
DD
R
EI0
PA0
C
EXT
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 i s controlled by the RC time constant. The
value of the C
voltage and with temperature.
is fixed but the internal pull-up value varies with the MCU power supply
ext
PU
) at different power
DD
AN1605/0403 1/7
1
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)
1V
2V
= 5.0 V 50 kΩ 120 kΩ 250 kΩ
DD
= 3.0 V - 160 kΩ -
DD
The consumption is measured for two different values of C
Table 2. Consumption for C
Average I
Setup No. V
(Volts)
DD
Consumption in
= 1 µF
ext
(mA )
DD
Run mode
Average I
DD
(µA)
Consum pt i on i n
Power saving
mode
, given in Table 2 and Table 3.
ext
Ext Interrupt
Interval (ms)
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
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
I
(µA)
DD
Consumption
when MCU is in
HALT
0.1
2/7
2
Table 3. Consumption for C
Setup No. VDD (Volts)
= 0.47 µF
ext
Average I
DD
(mA)
Consumption
in Run mode
Average IDD (µA)
Consumption in
Power saving
mode
Ext Interrupt
Interval (ms)
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
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:
I
(µA)
DD
Consumption
when MCU is in
HALT
0.1
■ The MCU inserts a delay of 256 C PU cycles to stabilize the internal RC, d uring 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
3/7
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