AN3404

General power management tips

Doc ID 018847 Rev 3

17/30

5.3

Dynamic control of pull-up resistor

Many applications have buttons which are used as a user interface. These buttons are

used in order to have a defined logic level on these I/Os when the button is off (not pressed).

Once the button is pressed, the I/O is grounded and it generates an additional current of ~

40 – 70 µA, drawn from the power supply. This current is negligible if the application is in

Run mode, but becomes very important for applications which run mainly in low power

modes.

For such low-power applications, the pull-up can be dynamically controlled. Once a button is

pressed, the related service routine is executed. This routine disables the pull-up and the

interrupt, in order to save consumption while the button is being pressed.

As the application has to check the button status regularly, the pull-up is enabled again

before any checks are made. If the button is continuously pressed, the process is repeated.

Once the button release is detected, the pull-up remains on.

For this task, an I/O can be left floating for a short time frame, given by a check period,

generating an extra consumption in the range of ~10 µA. Therefore, the total current

consumption for a button-press operation is lower than without such dynamic control.

5.4

Waiting loops/delays

In some cases, the application has to wait for an input from the user, communication

peripheral, or another external unit.

If there is no useful code to execute while waiting for an input, it is recommended to avoid

using either a typical delay loop or a polling loop.

●

Typical delay loop:

(delay = 0; delay < 0xFFFF; delay++)

_asm ("nop");

●

Polling loop:

SPI_DR = data; // start communication

while (!( SPI_SR & SPI_SR_TXE)); // wait for TXE bit set

A Wait mode woken up by an interrupt on the SPI transmitter empty (SPI_SR_TXE flag) is

advised by the following sequence:

●

Enable TXE interrupt:

SPI_ICR |= SPI_TXIE; // enable interrupt for Tx empty

An event is then generated after each byte is sent. The next data can be loaded in the Tx

buffer as follows:

SPI_DR = data; // start communication

_asm ("wfi"); //or wfi() if you include STM8TL5x.h file in your

project

The sequence described above uses WFI mode to wait for Tx buffer empty. A delay loop can

be replaced by a Wait mode woken up by a programmed timer interrupt or better an Active-

halt with AWU if no other peripheral is running. This can also be used for other

communication lines. For external sources (like interrupt from an I/O port), Halt mode can

also be considered.