UM1574 datasheet(30/48 Pages) STMICROELECTRONICS

30 / 48 page

Hardware and layout

UM1574

30/48

Doc ID 023645 Rev 1

The measurement timing is given in Figure 17. In Low power modes, the 2 K resistor is

connected when the U5 switch goes off after entering Low power mode. The Q13 output of

the counter allows connection of the 2 K resistor when the current IDD becomes very low.

Figure 17 shows how the counter and U5 ensure that, 150 ms after IDD_CNT_EN falling

edge, the shunt resistor R24 is connected between VDD_MCU and the power supply in

order to reduce the measurement range to 30 µA full scale (instead of 30 mA in Run mode).

Then after another 150 ms for current stabilization, R24 is shorted, the IDD measurement is

stored in C14, and the MCU is woken up. After wakeup, the MCU can measure the IDD

current corresponding to the Low power mode stored in C14.

Figure 17.

STM8AL board IDD Low power mode measurement timing diagram

●

Ibias current measurement procedure

In Low power mode the bias current of operational amplifier input (U8 pin 4) is not negligible

compared to IDD current (typical Ibias is ~240 nA). To obtain a reliable STM8AL3L68T IDD

measurement, it is mandatory to subtract the bias current from the IDD low power

measurement since MCU is not sinking the current. Ibias is measured during production test

and stored in the MCU EPROM. The demonstration software, Discover, uses this value to

display the correct IDD. The Ibias measurement procedure is part of the demonstration

software and can be launched if required.

The procedure for Ibias measurement (implemented in the package demo) is:

Power off the board (disconnect the USB cable).

Set jumper JP2 to OFF position (pins 1 and 2).

Push down USER2 button while powering on the board from the USB.

Wait at least 1 second before releasing USER2, the LCD displays the Ibias

measurement.

Power off the board (disconnect the USB cable).

Set jumper JP2 to ON position (pins 2 and 3). The Ibias value is now stored. The bias

current is then subtracted from the IDD measurement performed in Low power mode.

MCU mode

Run

Low power

Wake-up

Clear CNT

IDD_CNT_EN

Q13 = LOW_POWER_EN

Q14 = IDD_WAKEUP

Q14n = U4 pin 4

0ms

150 ms

300 ms

event

Wake-up

period

IDD

measurement

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