SIM225 Technical Reference Manual
32
© Serious Integrated, Inc.
SIM225_TRM_A1ENUS
Revision A1
USB DEVICE (“FUNCTION”) POWER
The SIM225 can be powered from the
+5V_USBF
signal present on the
USB Mini-B Device Connector
, the
Tag-Connect Programming Port
, the
PCB Edge Connector
and the
Baseboard Connector
. This input is
limited to draw a maximum of 500mA, typically from a PC or hub assuming it can supply sufficient current.
Verify the USB hub or PC can supply the
required power
to the SIM before connecting.
The USB Mini-B Power input pin (+5V_USBF) on various other connectors is directly connected
to the USB Mini-B power input:
connecting any of these simultaneously may damage your
SIM or even connected equipment such as a PC or USB Hub.
The 500mA limit on +5_USBF is lower than some variants, for example the SIM225-A00 with
0.7W audio amplifier/speaker, require for their complete functionality. Turning on some
features without sufficient power may cause a system reset. Ensure sufficient power from the
various power supply inputs before turning on these features.
LCD PANEL BACKLIGHT POWER
The
LCD Panel
has an array of LEDs supplying the backlight. This LED array requires approximately 16V at
up to 800mW to fully light. An
On Semiconductor CAT4139
boost regulator and associated circuitry
boosts the main power to this voltage, and can be pulse width modulated to vary the brightness (and
power required) of the LCD panel.
Backlight power is a significant portion of the SIM’s power consumption. At full brightness, the LCD
backlight typically consumes 800mW (50mA @ 16V). Working backwards through the CAT4139 (at about
85% efficiency), this represents a load of approximately 214mA on the
+5V_MAIN
supply. Software should
carefully manage the backlight to be powered on as infrequently and for as short a time as possible –
especially in battery powered systems or when operating from the USB power. Since both of these
conditions are detectable
, software can drop the brightness to less than 100% when operating from these
sources and even further reduce brightness as the software
detects the battery voltage
dropping.
Another technique is to turn off the backlight completely when the touch panel has not been touched for
some duration, for example 30 seconds, and then waking up the backlight when touched again or a
condition on the GUI needs attention. This mode often can create a long-term duty cycle of <10% and
extend battery life significantly.
The LCD backlight is enabled when RX
P11/BLEN
is driven high, which turns on the backlight power
boost circuit driving approximately 16.5V to flow to the backlight LEDs on the LCD. This enable signal has
a weak pull-down, so the backlight is off until the MCU pin is initialized, including during and directly after
system
RESET#
. Software algorithms can PWM this pin to enable backlight dimming. A PWM driven by a
typical 1 KHz clock with 16 PWM steps for a PWM net frequency of 64 Hz is generally sufficient and flicker
free with duty cycles from 0 to 100%. In no circumstances should the PWM clock exceed 16 KHz.
The
Serious Human Interface™ Platform
has the backlight driver included; setting the platform
glass backlight value to 0 to 100% automatically modulates the dimming circuit.
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