Active low woes

I'm trying to interface with a radio module (DNT900), and I'd like the ability for the Arduino to reset it. The device has an active low reset pin.

I've been running the DNT900 with the RESET pin disconnected and the device works fine, the device also works fine when the RESET is connected directly to 3.3v.

The issue is when I connect an Arduino pin via a 5v - 3.3v converter (CD4050) and set the pin HIGH or LOW, the device does nothing! I can't get any response on the serial line. I've also tried a 10k pulldown resistor connected from the arduino pin to GND.

Any suggestions on how to make this work would be appreciated.

Side question: How long would I have to set the line LOW to cause the device to reset?

Got a link to the DNT900?

CrossRoads: Got a link to the DNT900?

I sure do: http://www.rfm.com/products/data/dnt900dk_manual.pdf

Page 35 describes the pins.

I only see this - not much help about applying a hardware reset, or how much current is needed to pull it low. I would try a transistor like 2n2222a or bc547 for 100mS, see if that works.

3.7 Power-On Reset Requirements When applying power to the DNT900, the /RESET Pin 39 and the RADIO_TXD Pin 31 must be initially held low. The /RESET pin must be held low until the power supply voltage reaches 3.3 volts for 100 ms, and then set high. The RADIO_TXD must be held low an additional 10 ms after the /RESET pin goes high. RADIO_TXD is weakly pulled down with a 100K ohm resistor to meet the power-on reset requirement, unless this line is driven high by an external signal.

Why did you put a pull down resistor on an active low line? Use a pull up resistor and apply the required length reset.

I have to say I find the idea of a "pull down" resistor really weird. It's more termination than setting a known logic state.

I think he was just trying to find ways of getting a hardware reset to work.

CrossRoads: I think he was just trying to find ways of getting a hardware reset to work.

And my point was to forget the "pull down" resistor!

THERE IS NO SUCH THING AS A "PULL DOWN" RESISTOR!

sorry...

As for how long to hold the reset line low,

RTFM/RTFD!!!!

there will be a minimum time for the Reset line to be held.

I'll take a stab at "not long" Most chip's reset function is the fastest thing they do!

sinaptik:
The issue is when I connect an Arduino pin via a 5v - 3.3v converter (CD4050) and set the pin HIGH or LOW, the device does nothing! I can’t get any response on the serial line.

Do you mean it never works? It works until you pull the pin low, and high again? If so, for how long do you do it?

If it normally works with the reset pin disconnected, then it should at least work the first time (when you reset the Arduino) because the pins are high-impedance. That is, if you remove that resistor.