Hi,
What are the advantages of double diodes, one to GND, the other to VCC against the one only connected to GND?
Sorry for late reply.
Advantage .. or not...
It depends on the port you want to protect.
Example:
1)
See 74HC00 datasheet:
https://assets.nexperia.com/documents/data-sheet/74HC_HCT00.pdf
It defines the "max input clamp current" for V_IN= -0.5V ... VCC+0.5V
It is related to VCC.
If VCC= 3.3V you should not apply mor ethan 3.8V.
If power down: VCC=0V, then you should not apply more than 0.5V
--> Thus I use a schottky diode to VCC
2)
See 74
AHC00 datasheet:
https://assets.nexperia.com/documents/data-sheet/74AHC_AHCT00.pdf
It defines the "max input voltage" -0.5V ... 7.0V
It is
not related to VCC.
If VCC= 3.3V you may apply up to 7.0V. (5V input tolerant)
If power down: VCC=0V, then you still may apply up to 7.0V.
--> Thus I use a zener/TVS... or a schottky diode to VCC
Other advantages: fast, low threshold voltage, low capacitance
Conclusion:
With the schottky to VCC and a schottky to GND you are on the safe side.
Disadvantage of the schottky to GND:
There are circuits where a schottky to VCC is counter productive.
An example is: multiple RS485/RS422 devices on the same bus.
RS485 transceiver behave "neutral" = "high impedance" when not powered. Thus it´s possible to have some devices not powered up but still communication with other bus partners will work.
But if you use schottky diode to VCC on an unpowered RS485/RS422 bus, then it pulls down communication signals and the communication to all bus partners will fail.
Klaus