There are some expansion boards with a 3.3v regulator and diodes to clamp the voltages of the programming pins.
I don't think this will work as intended. The diodes are there to reduce the voltage on the programming pins to no more than 3.3v, however this would only work if the diodes have no forward voltage drop. As a typical diode has a voltage drop of about 0.6v the lines will go up to around 3.9v which is too high for safely programming a 3.3v device.
Wouldn't it be safer to use either 3.3v zener diodes to ground or the usual Mosfet voltage converter circuit?
Last edit: Tim Steele 2022-11-28
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This kind of circuits work because there's a series resistor that limits current.
The current is split between the diodes on the board an the internal protection diodes.
In turn this means that voltage cannot rise more than 0.5V-0.7V above VDD.
No need to change it.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Anonymous
Anonymous
-
2022-12-01
Having a 3.3V Zener to ground (with the resistor) will clamp the
voltage correctly, not 0.6V above Vdd.
This kind of circuits work because there's a series resistor that limits current.
The current is split between the diodes on the board an the internal protection diodes.
In turn this means that voltage cannot rise more than 0.5V-0.7V above VDD.
No need to change it.
IC inputs with this kind of specifications have two ESD protection diodes that clamp the voltage to VDD and GND. The input circuitry itself can withstand a voltage much higher than 3.3V.
So the only constraint is not to blow the diodes, and this is accomplished by limiting the inrush current with whatever method you like.
Limiting the voltage to VDD+0.3V is one of the methods, not the only one.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
There are some expansion boards with a 3.3v regulator and diodes to clamp the voltages of the programming pins.
I don't think this will work as intended. The diodes are there to reduce the voltage on the programming pins to no more than 3.3v, however this would only work if the diodes have no forward voltage drop. As a typical diode has a voltage drop of about 0.6v the lines will go up to around 3.9v which is too high for safely programming a 3.3v device.
Wouldn't it be safer to use either 3.3v zener diodes to ground or the usual Mosfet voltage converter circuit?
Last edit: Tim Steele 2022-11-28
This kind of circuits work because there's a series resistor that limits current.
The current is split between the diodes on the board an the internal protection diodes.
In turn this means that voltage cannot rise more than 0.5V-0.7V above VDD.
No need to change it.
Having a 3.3V Zener to ground (with the resistor) will clamp the
voltage correctly, not 0.6V above Vdd.
Tim
On Tue, 29 Nov 2022 at 21:48, Alberto Maccioni
albmcc@users.sourceforge.net wrote:
PIC16F1847 datasheet says pins can only tolerate a maximum of Vdd + 0.3V
IC inputs with this kind of specifications have two ESD protection diodes that clamp the voltage to VDD and GND. The input circuitry itself can withstand a voltage much higher than 3.3V.
So the only constraint is not to blow the diodes, and this is accomplished by limiting the inrush current with whatever method you like.
Limiting the voltage to VDD+0.3V is one of the methods, not the only one.