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Reset issue

Rajinder1268

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Hi,
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.
 

barry

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It's most certainly not the push pull circuitry. Read the data sheet. Vol= 0.4

You've either got a defective chip or something wrong on your layout. Without a schematic, that's all I can say.
 

FvM

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I agree that you can't pull down the push-pull output of 809 reset device. You can place a series resistor of e.g. 1 k at the output, also a series diode for the CBUS0 output to avoid blocking of the reset device.
 

Rajinder1268

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Hi all,
Sorry I have attached a diagram of what I am doing. The supervisory device will reset the microcontroller when VDD falls below the threshold of (2.9V).
I also need to reset the microcontroller from the CBUS0 of the FTDI, which is controlled via USB.
I think that I need a open drain supervisory device, so that I can make a wired-OR function, i.e. the supervisory or FTDI can reset the microcontroller.
--- Updated ---

Here is the diagram.
 

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KlausST

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Hi,

You don't need "OR" you need AND.
You may do this with pullup and diodes / open_drain (wired AND)
You may use a one part solution like a 74HC1G08

I prefer the wired AND solution because you are more flexible in adding parts, like a pushbutton or a capacitor for delay.

Klaus
 

Rajinder1268

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Hi,

You don't need "OR" you need AND.
You may do this with pullup and diodes / open_drain (wired AND)
You may use a one part solution like a 74HC1G08

I prefer the wired AND solution because you are more flexible in adding parts, like a pushbutton or a capacitor for delay.

Klaus
Hi,
I am going to try and use a open collector supervisory pin for pin. I need a pull up on the output. Not sure about the diodes you mentioned. Please could you explain the AND connection with reference to my block diagram.
--- Updated ---

If I use a open collector supervisory, is this what I require?
 

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Last edited:

KlausST

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Hi,

The direction of your diode is wrong.

Write a simple truth table to see your mistake.
(This is what I do when I´m not sure)

Klaus
 

barry

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That diode might screw things up. With ~0.7 V drop across the diode, plus the Vout-low of the 809, the voltage at the PIC might not be low enough to reset it. Maybe a Schottky diode would be better.

You could also put a voltage divider on the INPUT of the 809 with the bottom of the divider going to the C0 signal. The input current to the 809 is pretty low, so you could use fairly high resistances. This assumes VDD is not greater than the maximum allowed for C0.

But, stupid question: doesn’t the PiC have brownout detection, precluding the necessity of the 809?
 

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