[General] Pin state during reset

[keyboardcowboy]

This is a general question, and can apply to any micro-controller.
If I have a pin configured as output and connected to an led, how can I prevent the led from turning on briefly when the uC is turned on or reset. As the uC turns on (or is reset), all the pins are high, and then it starts to execute code, which makes the pin low and hence turn the led off. This can also apply to motors, they will turn for a very short time when the uC starts

 

[KlausST]

Hi,

As the uC turns on (or is reset), all the pins are high

If this really is the case.....then you should consider to switch ON the LED with a LOW at the port.
* connect the LED to VCC, so it is ON when port pin is low
* use an inverter between port an LED (logic, bjt, fet...)

Often port pins during RESET are not true HIGH -- but high Z, or pulled up then you may find other solutions.

If you need more help, please show schematic and say what microcontroller you use.

Klaus

 

[FvM]

As the uC turns on (or is reset), all the pins are high

Guess you are talking about a 8051 processor (or one of its derivates), which has a bit antique GPIO topology. The best way to operate its outputs in a safe manner is to use active low level, so all outputs are off by default.

Recent processors have initially tri-stated outputs. You can operate them active high or active low at your convenience, optionally forcing an inactive level by pull-down respectively pull-up resistors (plus a well-considered initialization order).