ThingMagic Mercury xPRESS Platform v1.7 - Manual (Page 121)

A D I V I S I O N O F T R I M B L E
Hardware Reference Guide 121
system voltage so the the other power sources (from the battery or USB line) will not be
drained through this interface.
The Modules are powered directly from this voltage, so it must be 5 V+/- 5% for the M6e
and between 3.4 to 5.25 V for the Micro.
If a Li-ion battery is also plugged in, the AC-adapter power source will be used to charge
it. If the AC adapter input voltage drops below 4.3V, the battery will power the system.
As much as 1A might be used to charge a battery. The M6e, when transmitting at full
power, can consume nearly 1.5 A, so the total current draw could be as much as 2.5 A.
There is no reverse power protection for the input AC line. This is usually a requirement if
the unit is to be powered by a vehicle (where, potentially, the vehicle battery could be
inadvertently installed with the polarities reversed.)
If both an AC adapter and a USB host connection are plugged in, the 5V from the USB
host will not be used to charge the battery or power the system (unless the voltage level
on the AC adapter drops to below 4.3 V).
USB Connection
The “USBOTG” port can be connected to either a USB host or USB client. The system
detects the connection type by the impedance to ground of the “ID” pin on the connector.
If this pin is grounded, the connected USB device is assumed to have the ability to supply
power to the system. If it is high impedance, the connection may need power from the
system. (The decision to supply power to the client USB port is made by the MCU, so can
be overridden by the user program.)
If the xPRESS motherboard is connected to a host, up to 500 mA of current at +5 V can
be obtained from the USB port. The MCU is capable of extending that limit to 1 A if it is
known that the USB host can handle this. There is no over-voltage protection on this line
as there is for the AC adapter input line. The operational limit of 5.25 V must be
maintained by the source of this voltage, as well as the safety limit of 6 VDC.
If the xPRESS motherboard is connected to a client, an IC (U6) provides over-current
protection and will turn off the USB port within 40 nsec if current limits are exceeded for
more than 22 msec. The current limit will normally be around 1.5 A when the xPRESS
motherboard is powered by an AC adapter.
Once the current threshold is exceeded, the regulator will only be reset if it is
powered down or if its “on/off” control line is toggled (this signal is controlled by the
MCU).
If a USB host connection is present at the same time as the Li-ion battery, then this
connection will be used to charge the battery. It also will power the xPRESS motherboard,