What is the diode going to?

[chopnhack]

Does anyone know what the diode is connected to at the top left hand corner?? The datasheet recommends this method of connection to protect the μcu, which makes sense with high voltage programming potentially damaging the chip. I'm just a little confused as to why its not labelled and what is it supposed to be?

I intended to use 1k Ω for R and 4.7k for R1 - does this make sense? The datasheet allows a lot of leeway for values.

 

[hevans1944]

It probably connects to Vdd to rapidly discharge C when Vdd goes to zero. See Note 1 in your diagram.

 

[chopnhack]

It probably connects to Vdd to rapidly discharge C when Vdd goes to zero. See Note 1 in your diagram.

Wow... I think I need to put the datasheet down...

 

[chopnhack]

While I was trying to make sense of the first schematic I stumbled across another method of protecting the pic/associated circuits from high programming voltage. The zener is rated at 5.1V, the square represents a pic mcu. From what I understand, when the ICSP is in use, higher programming voltages can be seen in the circuitry. If the CLR is taken low for whatever reason - those high voltages would come across Vdd through the circuit and damage it.

What do you think of this schematic?

 

[Arouse1973]

It probably connects to Vdd to rapidly discharge C when Vdd goes to zero. See Note 1 in your diagram.

I am thinking also R1 will prevent the Vpp line from the programmer drawing excess current. MCLR lines don't have the ESD protection diodes that other ports have because if using the Vpp line for ICP the programmer would be shorted out by this diode, which would only allow a Vpp voltage of VDD +0.3 V. The Vpp programming voltage could be 12 Volts.
Thanks
Adam

 

[chopnhack]

I am thinking also R1 will prevent the Vpp line from the programmer drawing excess current. MCLR lines don't have the ESD protection diodes that other ports have because if using the Vpp line for ICP the programmer would be shorted out by this diode, which would only allow a Vpp voltage of VDD +0.3 V. The Vpp programming voltage could be 12 Volts.
Thanks
Adam

Thanks Adam.

In the last schematic I posted, I think it works by providing similar safety against high programming voltage by clamping the voltage to no more than 5.1v (the zener that is mislabeled as 1n750). What are your thoughts on it?

 

[gorgon]

The zener and R1 is used to stop the programming voltage to feed back into Vdd from the ICSP via R2. The pushbutton is normally used to reset the mcu, and needs a pullup to Vdd to function correctly.

 

[chopnhack]

The zener and R1 is used to stop the programming voltage to feed back into Vdd from the ICSP via R2. The pushbutton is normally used to reset the mcu, and needs a pullup to Vdd to function correctly.

Thanks for confirming this Gorgon.
To clarify, as shown, is the pushbutton not pulled up to Vdd via R3 and R2?
From what I understood of the above schematic, MCLR stays high until pushbutton is depressed which then brings everything to ground potential.
Sorry, I am still learning a lot of this, can you confirm or explain further?

Thanks in advance!

 

[gorgon]

The R2 is the pullup resistor, since it is connected to the MCLR input. R3 and C1 forms a LP-filter to remove bounce glitches from the pushbutton, when activated. R3 will also protect the pushbutton from the high discharge current when shorting C1, this protect the contact points from 'welding' spots. C1 will also protect the MCLR input from noise, and unwanted resets because of that, and give a power on reset when powered up.