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Capacitive Touch BoosterPack Hardware
3.1
Driving the LEDs
The white center LED is connected to port P1.0 of the Value Line device and can be turned on by setting
this port as output. The other eight LEDs are multiplexed as shown in
Figure 7
. Four LEDs are connected
with the diode cathodes to the ports P1.4 to P1.7, and the other four LEDs are connected with the diode
anodes. All LEDs are connected to port P1.3 to either source or drain four LEDs at any one time. To drive
a particular set of these eight LEDs, two steps are required:
1. Port P1.3 acts as GND drain, so that the first nibble can drive LED1 to LED4 directly. The other LEDs
are not active during this time.
2. Port P1.3 acts as a VCC source to light LED5 to LED8. The pattern for the second nibble must be
inverted and set to port P1.4 to P1.7.
Figure 7. Schematic LED Multiplexing
Switching between modes can be controlled by a timer and has to be at least 100 times a second to
generate the illusion of a constant light pattern. To reduce glitches while switching the LED modes, it is
recommended to set the LED signals that are not being driven to input mode.
Figure 8
shows the signals
of all the LED driving pins required to light up LED1, LED3, LED6, and LED7. The mode on P1.3 can be
set before or after the LED settings, as long as the unused LEDs are set to input mode.
NOTE:
The current user experience implementation is not using a time-shared approach to drive the
LEDs, due to the shared CPU and timer resources utilized by the Capacitive Touch Sense
Library [7] functions and the UART transmissions.
Figure 8. Driving LEDs
15
SLAU337A
–
April 2011
–
Revised September 2011
430BOOST-SENSE1 - Capacitive Touch BoosterPack for the LaunchPad
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