Electronics > Beginners

Pulling a line up for a while after power-up

(1/1)

andri:
Bit of background: I have a WRT160NL router that has a TTL serial, and I have built a small circuit that interfaces 1-Wire devices. Unfortunately, this circuit echoes everything the router transmits back to itself, which confuses the hell out of the bootloader when the router is restarting (it thinks someone pressed a key on the serial console and drops into rescue mode).

Thus, I thought the obvious way to solve it would be to whip up something that pulls the RX line high for, say, 10-15 seconds after power-on giving time for the bootloader to do it's work and pass the control on to the Linux kernel; after that, the circuit may as well echo everything back again as it won't disturb anything critical anymore.

However... I've tried figuring out how to build this kind of circuit and can't manage to do it. I have a feeling that it shouldn't be too difficult to do, with a resistor and a cap to provide the timing (no need to be super precise here) + a couple of transistors to actually do the pull-up.

If anyone could lend a hand designing this, it'd be appreciated.

(I'm more interested the how, ie please don't slap a .png of a circuit that does it -- I want to learn!)

Of course, I could give up and rebuild the whole damn circuit around an ATtiny13 MCU or something similar, and do the whole thing in software. It however feels... wrong to waste a MCU for something this mundane.

Psi:
Sounds like a good use for a 555 timer in one shot mode.

ya can easy set that to 15seconds.

Zero999:
If the input is a Schmitt trigger with a high imput impedance, you could connect a capacitor between the line and +V and a resistor from the line to 0V. Failing that, there are many options: 555 timer (as mentioned above), comparator, 74HC14 (or single Schmitt inverter), 74HC04 (or dual inverter) or even a couple of transistors.

Navigation

[0] Message Index