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Reset issue

Rajinder

Jan 30, 2016
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Hi all,
I am using a ft232r usb UART device. I use cbus0 to control the reset line of a supervisory ic @3v3. The supervisory ic also connects to the reset line of a PIC microcontroller. The supervisory device is active low with a push pull output. It is a device from ON semi CAT809JTBI-GT3. I can't seem to pull the supervisory line to 0V to reset the PIC via USB control. It stays at around 2.5V.
I suspect it is because of the push pull circuitry. Can anyone assist.
 

bertus

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Hello,

The output should go down to about 0.3 Volts:
CAT803_D-_output_characteristics.png
Bertus
 

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Rajinder

Jan 30, 2016
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Hello,

The output should go down to about 0.3 Volts:
View attachment 57132
Bertus
Hi,
Yes I agree but it doesn't. The supervisor works fine i.e. if the voltage falls below the threshold it resets the PIC. However if I try and force the supervisor output (which is connected to the reset line of the PiC) from the CBUS0 of the ftdi device, it stays at 2.5V.. Is it because the supervisory is a push pull type?
 

Rajinder

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This is what I have. The supervisory circuit works and resets the micro once it falls below it's threshold level. I can't control using the CBUS0 line. I thought because the output of the supervisory is tied to VDD, hence trying to pull it low won't work. I am thinking of using an open collector supervisory with a pull up resistor. Which I can then pull low with the CBUS0 line. I might need to add a diode?
 

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Harald Kapp

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You cannot connect two outputs directly (reset from teh supervisory circuit and reset from the ftdi chip. In that case the high from the supervisory chip will pull against the low from the ftdi, resulting in the observed 2.5 V.
Methods to avoid this dilemma:
- use open drain outputs and a single pull-up resistor (wired or).
- use an AND gate
- use diodes and a pull-up resistor like so:
1669795996056.png
 

Rajinder

Jan 30, 2016
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You cannot connect two outputs directly (reset from teh supervisory circuit and reset from the ftdi chip. In that case the high from the supervisory chip will pull against the low from the ftdi, resulting in the observed 2.5 V.
Methods to avoid this dilemma:
- use open drain outputs and a single pull-up resistor (wired or).
- use an AND gate
- use diodes and a pull-up resistor like so:
View attachment 57142
Hi,
Thanks for your help. I am looking to change the supervisory to open collector with a pull up.
Would I still need the both the diodes?
If I connect the supervisory output directly to the microcontroller. Then just have one diode on the CBUS0 line. So the reset works under low voltage. Then I can also drive it low via the CBUS0 line. I will post up a diagram.
 

Rajinder

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I think based on what you mentioned. This is what I require? Would the forward voltage of the diodes matter, I think they might, are Schottky diodes best for this application?
 

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Rajinder

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Can I omit the diodes if I use an open drain supervisory device?
 

Harald Kapp

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I am looking to change the supervisory to open collector with a pull up.
Would I still need the both the diodes?
No.
This is what I require?
Exactly what I sketched.
If I connect the supervisory output directly to the microcontroller. Then just have one diode on the CBUS0 line.
No. And your schematic shows otherwise, using 2 diodes as required.
Would the forward voltage of the diodes matter, I think they might, are Schottky diodes best for this application?
Schottky is best, but a standard 1N4148 (for example) will also work in most cases.
Can I omit the diodes if I use an open drain supervisory device?
You can answer that question yourself by studying the info about open drain and wired or that I linked in post #6.
 

Rajinder

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No.

Exactly what I sketched.

No. And your schematic shows otherwise, using 2 diodes as required.

Schottky is best, but a standard 1N4148 (for example) will also work in most cases.

You can answer that question yourself by studying the info about open drain and wired or that I linked in post #6.
Thanks, I will try using the open collector supervisory first with no diodes as pointed above in your feedback.
 

Rajinder

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The supervisory will be a open drain type. However the FTDI is a GPIO which can be configured as an output or input. Hence my question about using a diode?
 

Harald Kapp

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You cannot combine push-pull and open drain without additional measures (e.g. diodes). Read the info on wired or.
 
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