Hello.
I have a problem in my board.
I was hoping that maybe someone here has already ran into such problem and could give me a clue for the source of this problem.
In the board there are two ICs - Microcontroller & USB-to-UART Bridge - which are both connected to the same 47 kOhm pull-up resistor that connects to 3.3V.
The 3.3V comes from a 3.3V regulator that's driven by a 5V USB (the USB connects to a PC).
The problem is that when I connect the USB to the board, then the two RESET pins reach 3.3V only 200uSec after the VDD reaches 3.3V (The voltage curve of these two pins is just like a capacitor voltage curve).
That causes every POR (power-up reset) to be recognized in the UC as an HW reset (and thats the problem).
When I connected each Reset pin to its own pull-up resistor (without connecting the two Reset pins), then the delay has dropped from 200uSec to below 100uSec and the Power-Up-Reset wasnt recognized in the UC as HW-Reset anymore.
Could you tell me please why connecting the two reset pins causes a 200uSec delay, and disattaching them from each other reduces the delay?
The reset pin of the USB-to-UART bridge is an open-drain pin, so I dont see how it could add capacitance to the reset pin of the UC.
Thank you very much guys.
I have a problem in my board.
I was hoping that maybe someone here has already ran into such problem and could give me a clue for the source of this problem.
In the board there are two ICs - Microcontroller & USB-to-UART Bridge - which are both connected to the same 47 kOhm pull-up resistor that connects to 3.3V.
The 3.3V comes from a 3.3V regulator that's driven by a 5V USB (the USB connects to a PC).
The problem is that when I connect the USB to the board, then the two RESET pins reach 3.3V only 200uSec after the VDD reaches 3.3V (The voltage curve of these two pins is just like a capacitor voltage curve).
That causes every POR (power-up reset) to be recognized in the UC as an HW reset (and thats the problem).
When I connected each Reset pin to its own pull-up resistor (without connecting the two Reset pins), then the delay has dropped from 200uSec to below 100uSec and the Power-Up-Reset wasnt recognized in the UC as HW-Reset anymore.
Could you tell me please why connecting the two reset pins causes a 200uSec delay, and disattaching them from each other reduces the delay?
The reset pin of the USB-to-UART bridge is an open-drain pin, so I dont see how it could add capacitance to the reset pin of the UC.
Thank you very much guys.