Section A.2 - Application Notes.Design Examples
Version 1.3 - 2009-12
44
A.2.3.3 GREEN/Switch-1
signal
The GREEN/Switch-1 signal is a multiplexed signal:
-
GREEN
- Logic green LED signal (see Chapter 3, Table 8). Becomes valid 500ms after start
up.
-
Switch-1 -
The module reads the status of the signal at startup to decide if the serial
interface settings shall be restored (LOW) (see the Serial Port Adapter AT Commands
specification for more details). The input signal must be stable for the first 500ms after
startup (after reset/power on reset).
The signal is internally pulled-up (20k
Ω
) to 3.0V for NOT restoring settings to default.
Note! The internal 20k
Ω
Ω
Ω
Ω
pull-up is smaller than on previous product generations from
connectBlue. This requires a smaller pull-down resistor (4.7k
Ω
Ω
Ω
Ω
) for a low state.
If a LED is used and a switch is required to be able to restore the settings, a high impendence
buffer need to prevent the signal from being pulled-up via the LED. A 74HC08 logic gate can be
used in a 3/3.3V system (see Figure 21) and a 74HCT08 logic gate in a 5V system (see Figure
22).
Both gates are available in a single gate package (74AHC1G08/74AHCT1G08).
Sometimes, over time, switch contacts can get an oxide layer. This may cause the closed switch
resistance to become too high to sink the signal to logic LOW (the signal is internally pulled-up
to 20k
Ω
). A design that prevents this can be found in Figure 26.
Figure 21: Switch-1 used together with a
green LED in a 3.3V host system. The status
of the signal can also be used as input to a
host controller. Notice the 4.7k
Ω
Ω
Ω
Ω
serial
resistor.
Figure 22: Switch-1 used together with a green
LED in a 5V host system. The status of the
signal can also be used as input to a host
controller. Notice the 4.7k
Ω
Ω
Ω
Ω
serial resistor.
The buffer can be removed in a 3.3V system if no restore switch is required (see Figure 23).
In a 5V system the high state output (3.0V) from the module is not enough to turn off the LED,
so a 74HCT08 buffer can be used to raise the high level to 5V (see Figure 24).
The 74HCT08 gate is also available in a single gate package (74AHCT1G08).