SN74HC165: QH ouput voltage abnormal

Jerry Hsueh

Expert 2730 points

Part Number: SN74HC165

QH pin of SN74HC165PWT will jump from high level to low level (normally high level).

When the input of SN74HC165 parallel port works normally, it is 10K resistance pull-up(CheckD1~D10).

Is there any risk of damage to the chip by hot plugging and grounding these parallel ports during detection?

Or there are other possible reasons for abnormal QH voltage.

By default, when all pins of CheckD1-CheckD10 are pulled up (no pins are pulled to DGND), the output of SHI_QH2 should always output high level;

while the output of abnormal machine (U2 abnormality), SHI_QH2 has low level; the following figure is the waveform captured by the FPGA terminal.

Regards,

Jerry

over 5 years ago

0 Emrys Maier over 5 years ago

TI__Guru* 96681 points

Hi Jerry,
Yes, there is a risk of damage to the chip by hot plugging and grounding the parallel ports of the device. The outputs should never be set to output while also being shorted to ground - this could generate excessive current and damage the device.

Hot-plugging applications also typically have a higher likelihood of ESD damage -- if the device is exposed to human interaction (via a port, for example), then system-level ESD protection should be added.

0 Jerry Hsueh over 5 years ago in reply to Emrys Maier

Expert 2730 points

Hi Emrys,

Are there any protective solutions recommended for these pins?
Thank you very much.

Regards,
Jerry

0 Emrys Maier over 5 years ago in reply to Jerry Hsueh

TI__Guru* 96681 points

The way to protect them is to not short them to ground while they are active, and to have system level ESD protection at any external connectors.

If there is no way to prevent shorting to ground, a series resistor can be used to limit current (Ohm's Law: V=I*R)

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SN74HC165: QH ouput voltage abnormal