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i am working on some CPLD-controlled SPI devices that will be daisy-chained together. I will be using 74VHCT126AFTTR tri-state buffers to disconnect the SPI and Slave Select lines on the device from the daisy-chaining cable.

I have now a few questions:

The 74VHCT126AFTTR buffer will be powered by a 5V power supply line that is shared with the devices. With this voltage supply, will this buffer a 3.3V logic level SPI signal to a 3.3V logic level on the buffer's output?

When the VCC line on the buffer is set to ground will this affect the input lines of the buffer significantly? Could this make a problem for the SPI communication of other devices on the same BUS?

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    \$\begingroup\$ A diagram showing where you're using the HCT126 in your system (what is driving it and what is it driving) would help make your question more clear. \$\endgroup\$ – The Photon Mar 25 at 3:20
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The 74VHCT126AFTTR buffer will be powered by a 5V power supply line that is shared with the devices. With this voltage supply, will this buffer a 3.3V logic level SPI signal to a 3.3V logic level on the buffer's output?

The HCT family has CMOS outputs but TTL-compatible inputs.

That means it has a lower voltage input threshold. Any input above ~1 V will be interpreted as high, which means you can drive it with 3 V logic, even if it's powered by 5 V.

But its output is a 5 V CMOS signal, with (per the Panasonic datasheet) a minimum voltage of 4.5 V when high. So if you are using this to drive a load powered by 3.3 V, you'll need to be sure the load's inputs are 5-V tolerant.

When the VCC line on the buffer is set to ground will this affect the input lines of the buffer significantly?

According to the datasheet,

Input protection and output circuit ensure that 0 to 5.5 V can be applied to the input and output (Note) pins without regard to the supply voltage...Note: Output in off-state

so you should be okay, provided you hold the G pin low on the '126.

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  • \$\begingroup\$ Thanks for the answer. Would a sn74ls126 buffer then provide 3.3V logic level on its output? My device is not suitable for 5V logic. \$\endgroup\$ – Christoph Tönnis Mar 25 at 3:43
  • \$\begingroup\$ @Christoph, The LS126 has a 5 V TTL output. But that might not damage some 3.3-V-powered loads. You'd need to look at the datasheets of all the parts involved to be sure. Also, I doubt the LS126 will have the same ability to not load the bus when not powered on as the HCT126. \$\endgroup\$ – The Photon Mar 25 at 4:00

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