SN74LVC08A: SN74LVC08A --misoperation --when Vin=1.5V

Andrew Liu74

Part Number: SN74LVC08A
Other Parts Discussed in Thread: SN74AUP1T08, SN74HCS08, TLV7011

the MCU output driect connect to SN74LVC08A input pin ,when the mcu init start ,there mcu has 1.5V square output , then the SN74LVC08A will have 1.5V square output ,VCC of SN74LVC08A is 3.3V ,

SO the VIH shoud be 2V, if there any way to solve the misoperation , many thanks !

the picture 1 is input and output waveform ,the picture 2 is input and VCC waveform ,about 1.5V .

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 317260 points

Voltages between V_IL (0.8 V) and V_IH (2 V) are invalid, and might damage the device (see [FAQ] How does a slow or floating input affect a CMOS device?).

If 1.5 V should be interpreted as high, you need a level shifter, or an AND gate with lower input thresholds like the SN74AUP1T08.

0 Andrew Liu74 over 5 years ago in reply to Clemens Ladisch

Intellectual 970 points

Dear Clemens

the 1.5 V should be interpreted as low , if there any way to realize it ,many thanks ~

0 Chad Crosby over 5 years ago in reply to Andrew Liu74

TI__Expert 6680 points

Hi Andrew,

Section 6.3 of the Datasheet gives the VIH & VIL spec for this device. The maximum voltage that this device will be able to interpret as a LOW is 0.8V. You will either need to use a different logic device (maybe the SN74HCS08) or change your output signal from your MCU.

Thanks!

Chad Crosby

0 Clemens Ladisch over 5 years ago in reply to Chad Crosby

Guru 317260 points

At V_CC = 3.3 V, the SN74HCS08's positive switching threshold could be as low as 1.22 V.

To get a switching threshold of 2 V, you have to add a comparator like the TLV7011.

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SN74LVC08A: SN74LVC08A --misoperation --when Vin=1.5V