Design recommendations Cellular component firmware updates
Digi XBee3 Cellular LTE-M Global Smart Modem User Guide
64
Cellular component firmware updates
Even if you do not plan to use the USB interface, we strongly recommend you provide a way to access
the USB to support direct firmware updates of the cellular modem.
Even if you do not plan to use the USB interface (Pin 7 and 8), we strongly recommend you provide a
way to access the USB pins (Pin 7 and 8) to support direct firmware updates of the Cellular modem.
CAUTION! If you do not provide access to these USB pins, you may be unable to perform
cellular component firmware updates.
One way to provide access to the USB interface is to connect the USB pins to a header or USB
connector on the host design; see USB Direct design for more information. At a minimum you should
connect pins 7 and 8 to test points so they are easy to wire to a connector if necessary.
If you are using the USB pins for other purposes you must provide a way to disconnect those
interfaces during USB operation, such as using zero ohm resistors.
USB Direct design
Connect USB data pins 7 and 8 (USB_D+ and USB_D- respectively) on the XBee Smart Modem to the
appropriate pins on the host processor or connector. You may optionally connect pin 6 to VBUS.
Note Although pin 6 is 5 V tolerant, for compatibility with other XBee devices we recommend not
driving this line above 3.3 V.
See USB direct mode for more information on how USB is enabled and disabled.
The USB interface follows the USB 2.0 high speed specification and data lines must be designed as a
90 Ω differential pair. Both traces should be the same length and the overall trace length should be
kept as short as possible. If you are connecting to a USB connector then we recommend that you
employ an ESD suppressor appropriate for USB 2.0 HS (for example the Littelfuse PESD0402-140).
Other than the ESD suppressor, do not place additional components in the signal paths. The
suppressor is not needed if you are connecting to an on-board host processor.
CAUTION! If you use pins 7 and 8 for USB you must dedicate the pins to that interface.
Adding any traces to the main USB trace and/or connecting those traces to other
components will introduce signal integrity issues, which may result in unreliable USB
operation.
Power supply considerations
Note XBee LTE-M parts with an early revision of the microcontroller unit (MCU) may experience an
issue recovering from brownouts under rare conditions. See Brownout issue for details on how to
avoid this issue.
When considering a power supply, use the following design practices.
1. Power supply ripple should be less than 75 mV peak to peak.
2. The power supply should be capable of providing a minimum of 750 mAat 3.3 V (2.5 W).
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