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The tiny chip that powers Montreal subway tickets
To use the Montreal subway (the Métro), you tap a paper ticket against the turnstile and it opens. The ticket works through a system called ...
The die was covered with a passivation layer to protect its surface, a sandwich of silicon nitride and PSG (phosphosilicate glass) 1.1 µm thick according to the datasheet. 10 To send data back, the tag switches its load on and off at 848 kHz (1/16 of the carrier frequency), producing a subcarrier on top of the reader's signal. First, much of the chip's area is occupied by analog circuitry and the four bond pads, so shrinking the digital logic won't reduce the overall size much.
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