SN74HCS72-Q1: SN74HCS72-Q1 input and output issue

Sherry Liang

Part Number: SN74HCS72-Q1
Other Parts Discussed in Thread: SN74LVC2G74

Dear team,

Recently my customer wants to use our SN74HCS72-Q1 to replace NLV1403BDR2G, and they have two questions about our device.

1. Before they use NLV1403BDR2G, when this device powers down, and SET pin is high, Q pin output will be high because every pin's diode as below.

Does our SN74HCS72-Q1 have the same phenomenon? When SN74HCS72-Q1 powers off, what is the Q pin output under PRE pin active?

2. When NLV1403BDR2G powers up, Q pin will output a high level even though no input as below. Does our SN74HCS72-Q1 have the same phenomenon?

Thanks & Best Regards,

Sherry

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 281510 points

1. All HC/HCS devices have these ESD protection diodes. See [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)? and [FAQ] Is the reliability of a logic device affected by applying an input voltage larger than the supply (VCC)?

Consider using a device with overvoltage-tolerant inputs, like the SN74LVC2G74.

2. The initial output state is undefined, and might be high. See [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register)

0 Sherry Liang over 4 years ago in reply to Clemens Ladisch

TI__Genius 10255 points

Hi Ladisch,

Thanks for your reply!

About the first question, I still don't know the answer. When powers off, PRE pin is low level(active), what is the Q pin output? 0 or 1?

Thanks & Best Regards,

Sherry

0 Clemens Ladisch over 4 years ago in reply to Sherry Liang

Guru 281510 points

When the supply voltage is below the minimum recommended operating condition, correct operation is not guaranteed. In fact, without any power, operation is not possible at all; the output drives neither how nor low.

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SN74HCS72-Q1: SN74HCS72-Q1 input and output issue