Exploring the Raspberry Pi 2 with C++ by Warren Gay

Author:Warren Gay
Language: eng
Format: epub, pdf
Publisher: Apress, Berkeley, CA

If the input should go negative, however, the diode D1 begins to conduct and shunts current to ground (conduction would begin at about –0.6 V). Resistor R1 limits the current flow to avoid damaging the protection diode. This is the mechanism for discharging negative static charges. Likewise, if the voltage were to rise above +VDD, then current would begin to flow from the input through R1 into diode D2 to +VDD (D2 conduction begins at about +VDD + 0.6V). This time, R1 limits the current flow through D2. This simple mechanism keeps the MOSFET gate voltage within the range of Gnd (VSS) minus 0.6V to VDD plus 0.6V.

From a design standpoint, you should not count on R1 or the diodes for handling extreme signal swings. These are simply there to prevent static electric charges from damaging the device. If you have sloppy signal swings above or below the the gate limits, you should provide your own signal conditioning. The reason is simply that the the diodes or resistor might not handle the required current levels. For example, Broadcom does not provide current handling specifications of the GPIO protection diodes. They don’t want you to base your design on them.

Some of the CMOS 4000 series chips that you might use to interface with the Raspberry Pi use a zener diode in place of D1 or D2. This allows an extended input signal range. The CD4049 IC, for example, can be operated from a +3.3V supply and yet permit its input signal to be as high as +15V. This information is published in the component’s datasheet.

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