Resetting a slave from master mcu

[Demon]

I have a reset button on the master PIC, and I'd like to reset all the slaves internally. This is what I have so far:




I found formulas to calculate the base resistor, like this one:

https://breadboardcircuits.com/how-transistors-work/


But they're dependent on preliminary calculations done for the load current, and I have no clue how that is calculated with a PIC pin. I see several current loads in the datasheet for the 16F1933, but no clue which is the appropriate one to use in the formulas.

[Ioannis]

The main Load Current for the PNP transistor will come from the 10K, R5 resistor, that goes to +Vdd and not from the PIC /MCLR pin. Do not worry about that.

Ioannis

[Demon]

I had gotten that circuit from "somewhere", but I can't find any more. So, I'm looking at this guy:

https://www.electro-tech-online.com/...d-reset.17256/

He's using a NPN transistor, and I notice he's missing a pull-down on the base of the NPN.

I added labels to make it clear what attaches where, and I end up with this:




Do you see a problem if I have the programmer on the slave PIC at the same time as the pin from master PIC is connected?

I can always unconnect everything when I program, but I know "one day", I'm gonna rush and program with everything connected.

[Ioannis]

The base of the transistor is grounded through R6. The other end of R6 is Vss, meaning ground.

To Reset the system you have to feed high to the transistor in this circuit.

I am a bit worry about the collector that is connected to a 100 ohm and this to the programmer. In programming, there might be 13 or more volts. At this time you have to be sure that the transistor will be in OFF state, that is [/MCLR Slave from master PIC] be low. If you can be sure of this then I see no problem.

Ioannis

[Demon]

Yeah, this seems so sketchy to me. I'm thinking it would be safer to be able to connect only one at a time; either the programmer, or the master PIC.

[Demon]

I ended up with this design, using tri-state pin on master to reset all the slaves when required. This will not be used for programming the slaves, only resets.




It's simple, uses a single general I/O pin on master, and no extra components on slaves.

I include that 100R res on slaves "just in case" I need them one day with my Pik-Kit.


EDIT: For those that want to ICSP as well, look at the SN7407, and thank Mr E for the info (master on Input A, slave on Output Y)

[Ioannis]

What drives the MCLR pin of the slaves? A PIC output?

I am asking, because the MCLR pin needs a level of 0,2*Vdd to see LOW (for the 16F177x series, but I guess it is the same for others).

It depends on output characteristics if you are within limits of the MCLR. So be sure that what you think as low is actually sensed as low on the MCLR pins.

Ioannis

[amgen]

you could do in software..... send out to one or all listeners a command to goto a restart routine like.... which "provides a way to execute a MCLR in software"
,
,
,
restarting:
@ RESET

[Demon]

you could do in software..... send out to one or all listeners a command to goto a restart routine like.... which "provides a way to execute a MCLR in software"
,
,
,
restarting:
@ RESET

But will that work if the slave is caught in a loop or some such?

I'm aiming for a complete reset with no decision from the slave.

[Demon]

What drives the MCLR pin of the slaves? A PIC output? ...

A tri-state general I/O pin on the master.

Output low = reset
Input = do nothing

...

I am asking, because the MCLR pin needs a level of 0,2*Vdd to see LOW (for the 16F177x series, but I guess it is the same for others).

It depends on output characteristics if you are within limits of the MCLR. So be sure that what you think as low is actually sensed as low on the MCLR pins.

Ioannis

I can't say. This is all theory for now, but is supported by posts on google and a forum elder, Mr E.

So I blame him if my house burns down.

[amgen]

But will that work if the slave is caught in a loop or some such?

I'm aiming for a complete reset with no decision from the slave.

could do.........
1)
'
restarting:
pause 1000 ............... wait a second or 2 to reset all
@ RESET
'
or 2)................
if all slaves are on same 5 or 3 volt supply, then use a small NC (normally closed) relay to turn off power for few seconds to restart all slaves