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HUAWEI EM820W HSPA+ PC Embedded Module
Hardware Guide Description of the Application Interfaces
Issue 01 (2010-12-31)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
15
Table 3-11 LED_WWAN signal
Pins
Name Description Additional
Description
Direction to
Module
42 LED_WWAN Active-low LED signal
indicating the status of
the module.
L: Light on
H: Light off
Output
This signal is used to display the state of WWAN. The reference circuit diagram is
shown in the following figure.
Figure 3-4 LED_WWAN# signal reference circuit diagram
Normally, when the HUAWEI module is enabled, the LED is on, and when disabled, the LED is
off. The wink mode of the LED can be customized according to the requirement of the client.
For resistance of R placed on PC, choose the value that satisfies the following
equation:
I
F*R+VF+IF*100 =VCC
V
F: Forward Voltage
I
F: Forward current
Take the LED 19-213/GVC-AMNB/3T as an example (its manufacturer is Everlight
Electronics., Ltd. and the website is http://www.everlight.com). Figure 3-5 shows its
I
F
-V
F
curves. If VCC is 3.3 V and the desired current through the LED I
F
is 3 mA, then
the voltage of the LED V
F
is 1.5 V. According to I
F
-V
F
curves, the corresponding value
for resistance of R is (3.3 - 0.003 x 100 - 1.5) / 0.003 = 500 Ω.
The brightness of the LED depends on the current value, and for most of the
indicator lights the current between 2 mA to 5 mA is adequate.