Specifications
7
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DESIGN NOTES
This design note from Freescale Semiconductor discusses how to
use the latest Lighting Reference Design kit for LED lighting, see
page 20, to start new designs and optimise LED settings using
either the Digitally-Addressable Lighting Interface (DALI) or
DMX512-A wired protocols, or ZigBee® wireless control.
The Lighting Reference Design kit is being upgraded to help designers
create innovative solutions as markets for solid-state lighting continue
evolving rapidly. The features added will include support for wireless
LED control, enhanced design software, and integration with the
Tower System development platform (Figure 1). The new hardware
being created for the Tower System will include a controller module
built using a Kinetis ARM® Cortex™-M4 MCU, a DALI/DMX512-A/Zigbee
communications board, and LED-driver boards featuring the MC13213
MCU and integrated IEEE 802.15.4 transceiver. The controller will
manage multiple LED-driver boards, using either DALI or DMX512-A
wired protocols or the ZigBee based wireless Home Automation
Network (HAN) prole within the ZigBee specication. A switch on
the LED-communications board will communicate the choice of DALI,
DMX512-A or ZigBee to the Tower-based lighting-control application.
The DALI specication allows for a theoretical maximum of 125 LEDs to
be attached, whereas the DMX512-A specication allows control of up
to 512 LEDs. The ZigBee specication allows for 250 LEDs, while ZigBee
Pro supports 1000 LEDs. Once the communication protocol is chosen
the lighting controller will scan the system to see how many LEDs are
present. During this scan each LED-driver board reports the type and
number of LEDs present. For the DALI-based system, which operates
at 1200bits/s, this can take several seconds. For DMX512-A systems,
which operate at 250kbits/s, the scan is almost instantaneous.
For the ZigBee based systems, which also operate at 250kbits/s, the
scan again is almost instantaneous. Each LED-driver card will have
ZigBee Full-Function Device (FFD) router capability, allowing each LED-
based light to communicate wirelessly with its nearest neighbour. This
extends the potential wireless coverage over hundreds or thousands
of metres from the original lighting-controller system.
After the lighting controller discovers all the LED subsystems, the eGUI
touchscreen-LCD panel will allow LEDs to be selected individually
to open a new window enabling the colour and luminance for the
Getting the most from the new lighting reference design kit
selected LED to be adjusted using one of three techniques. The
default window, when an LED is selected, will oer sRGB colour-space
control, allowing a colour balance to be selected by touching within
the colour square and adjusting the slider to the right of the colour
square. A translation from sRGB colour space to CIE 1931 colour space
coordinates will also be calculated and displayed.
The next tab, CIE 1931 Colour Space, is selected, will display the standard
horseshoe-shaped CIE colour-space chart. A smaller triangle within this
colour space shows the gamut of colours that each particular LED or
LEDs can support.
As before, colour selection will be made by touching within the colour
space and then varying the luminance with the slider provided on the
right of the screen. Colour-space and luminance values may also be
entered directly via x, y and Y elds presented alongside the CIE colour-
space chart.
The third tab will enable direct control of each LED by setting values
for PWM dimming signals on a per-channel basis. This can only be
available when using DMX512-A or ZigBee lighting protocols, as
existing DALI protocols only allow acknowledgements - not data – to
be sent from a light. Although these will be represented as 8-bit values
in the eGUI they are actually 16-bit values within the MC13213 MCU,
and will produce smooth colour and luminance control of each LED.
The reference design will also support LED thermal management. Each
LED daughter board will position a thermistor as close as possible
to the LED array. Real-time thermistor data collected about once per
second via the MC13213’s ADC will allow the LED-controller board
to adjust the power delivered to each LED and so maintain junction
temperatures within predetermined limits, preserving the LED’s
lifespan.
Again for DMX512-A and ZigBee systems only, a fourth tab within the
eGUI will allow this sensor data to be read back and displayed. This data
feedback will use the DMX512-A Remote Device Management (RDM)
protocol and ZigBee Home Automation Network (HAN) protocol, both
of which will be supported in this update to the lighting reference
design.
Freescale
Semiconductor
Fig. 1: The upgraded Lighting Reference Design kit will be used
with the Tower System environment
Fig. 2: Sensor feedback with DMX512A- or ZigBee-based control