Preventing Electrical Damage in ThingMagic Nano Modules

Dan Ratner

7/21/2016

Since the release of the Nano module, we have had a few reports of Nano modules ceasing to work

properly after weeks or months of successful operation in customer-designed readers. The two must

common symptoms are:

 The module fails to initialize and will not communicate with the host

 The module will initialize, but presents a “firmware CRC error” (Error number 0200h) to the host

indicating that the application firmware is corrupted.

We believe the cause of these failures is memory corruption resulting from the circuitry around the

Nano not following the interface guidelines we give in our User Guide, which are based on requirements

from the supplier of the processor IC we use. There are two areas are of particular concern:

1. Input lines remaining high when the Nano module is turned off

2. Programming pin SWCLK allowed to float if not used

Input Line Requirements

In the Nano Design Guide, we give the following requirements for input lines. (The relevant clause is

underlined for emphasis.)

Input High-level Voltage

1.9 V min to indicate high state; 3.7 V max when module is powered up, no

more than 0.3 V higher than Vout when module is turned off to prevent

damage.

“Vout” is the DC voltage that the Nano sources on pin 10 and is nominally 3.3V as long as the DC input

voltage (pins 16 and 17) is greater than 1.8V.

This input voltage restriction applies to both the serial interface input line (“receive” side) and the GPIO

lines. Our recommended carrier board schematic diagram shows a buffer circuit that would enforce this

requirement for the RX input line. We did not take steps to protect the GPIO lines on the carrier board

because they can be used as either inputs or outputs depending on how the Nano is programmed.

We have discovered that the most common “damage” that is caused by violating this requirement is

corruption of the bootloader or application memory. Application memory corruption causes the

module’s bootloader to report a firmware CRC error, indicating that the firmware is not operational. If

Bootloader memory is corrupted, the module may not respond at all. The altered memory cannot be

repaired through user-accessible module interfaces, so the module cannot be repaired in the field.

Options for the way Nanos are shut down (by DC power or Shutdown line) and by the way the Nano and

input drivers are powered (by an independent supply or the same supply) leads to three solutions,

depending on the use case:

Case 1: If the Nano is powered by the same source that powers the circuitry that drives the

input lines to the Nano (so that everything is powered up or down together), and the Shutdown

line is not used to power off the Nano, there should be no potential for the input lines to remain

high when the Nano is powered off and the Nano should never experience this memory

corruption issue. See Figure 1.

Figure 1

Case 2: If the Nano is powered by a different circuit than the logic that drives the RX and GPI

lines, so that the Nano’s power could go down while the voltage to the input lines remains high,

the memory corruption issue could occur (regardless of whether the Nano is powered down

through dropping the DC supply voltage or using the Shutdown line.) See Figure 2.