SIM225 Technical Reference Manual
33
© Serious Integrated, Inc.
SIM225_TRM_A1ENUS
Revision A1
USB HOST POWER
In SIM225 variants with LiPo support (such as the SIM225-A00), there is no assurance that the main
operating power source is +5VDC; often as the battery drains, the main supply voltage will drop below 4.0
volts. To ensure adequate power to the
USB A Host Connector
these variants have a
TPS61240 3.5-MHz
High Efficiency Step-Up Converter
which boosts whatever voltage is present on
+5V_MAIN
into 5V for the
USB A Host Connector
.
This supply is enabled by asserting the
USB0_VBUSEN
signal high on RX MCU port
P16
, which is
connected to the TPS61240’s
EN
(enable) signal. The 5V Host power signal is designated on the
schematics
as
+5V_USBH
. The enable is pulled low during reset and remains low (disabled) until the port
pin is configured by software explicitly to drive high.
The
TPS61240
delivers 5V at up to 150mA to the USB Host connector, and converts at about 90%
efficiency from the incoming power supply. This 150mA is more than sufficient for most USB thumb
drives, and some keyboards and mice, but is unlikely to be sufficient for non-powered USB hard drives,
printers, etc. Keep this simple calculation in mind: the SIM power plus the power in the USB device
attached to the Host connector cannot be greater than the input power.
CLOCK/CALENDAR BATTERY POWER
Some SIM225 variants have a 20mm coin cell holder designed to accommodate a
common CR2032-type 3V battery. This is not designed to be a rechargeable battery,
nor does any circuit on the SIM225 supply power to charge this battery. The only
purpose and connection of this coin cell battery is to provide backup power to the RX
MCU’s Real Time Clock peripheral to keep the clock/calendar running in the event that
all other power sources are removed. Consult the
RX63N/RX631
datasheet for exact
specifications, but this battery can potentially keep the clock keeping time for several years without
replacement. The RTCC circuit in the RX MCU automatically switches to use the coin cell power only when
main power is not available, so in a system that normally has main power applied, the coin cell battery is
used infrequently. See the
Clock/Calendar
section for more information.
On SIM225 variants without the coin cell holder, the MCU’s clock/calendar must be re-set after every time
power is lost and re-applied to the SIM. In many system architectures, the time/date is available in
another portion of the system, and using communications methods the remote time/date can be
retrieved at power-up time and programmed in to the MCU’s RTCC registers.
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