SN74AVC4T245-Q1: when input pin(pin 2B2) is floating, the output pin (pin 2A2) is always high.

gavin zhang

Part Number: SN74AVC4T245-Q1
Other Parts Discussed in Thread: SN74AVC4T245

Dear TI support team,

Our SN74AVC4T245-Q1 schematic is as follows,

On one board, after I remove R3 and R6( that means the input pin and out pin are floating), the voltage of pin 2A2(output pin) is always high. We have manufactured more than 200 boards, only one board has this phenomenon, pin 2A2 on other boards are always low.

So why pin 2A2 is always high when pin 2A2 and 2B2 are floating on this board? why pin 2A2 on other boards are low?

Thanks

Gavin

9 months ago

0 Clemens Ladisch 9 months ago

Guru 271570 points

You must never try to do this. When an input is floating, its value is undefined; it might be low, or high, or oscillating. Also, this will increase power consumption: [FAQ] How does a slow or floating input affect a CMOS device?

0 gavin zhang 9 months ago in reply to Clemens Ladisch

Prodigy 20 points

Under the following test condition:

1)R6 and R3 are soldered,

2)MCU drive pin 2B2 always low

the waveform tested on pin 2A2 is as follows:

the pin 2A2 is high first, after 200ms later, pin 2A2 is oscillating, and then it is low.

Questions：

1)pin 2B2 is always low, why pin 2A2 can be high and oscillating?

2)only one chip has this phenomenon, why there is difference between chips?

Thanks

0 Clemens Ladisch 9 months ago in reply to gavin zhang

Guru 271570 points

Please check that the 2B2, /OE, and DIR pins have the correct voltage. If they do, then this looks as if that one chip got damaged.

0 gavin zhang 9 months ago in reply to Clemens Ladisch

Prodigy 20 points

ok, will double check.

other questions:

if input is floating, whether the output will be high on one chip, but be low on another chip?

If input is floating, whether the output will be low for most SN74AVC4T245 chips?

Thanks

0 Clemens Ladisch 9 months ago in reply to gavin zhang

Guru 271570 points

CMOS inputs have a small capacitance, so they tend to keep the old value for a short time. But their impedance is so high that they will pick up any noise. The value of a floating input depends on many random circumstances, so it is unpredictable.

Logic

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SN74AVC4T245-Q1: when input pin(pin 2B2) is floating, the output pin (pin 2A2) is always high.