Is there a reason software debouncing won't work?

You should be able to edge trigger when the button
is pressed and then ignore it for a while thereafter.
Compared to the bounce time, even rapid pressing
of a button will be easily detected.

Mike H.

On 10/25/06, Rolf <learr@rogers.com> wrote:
> Hi all.
>
> I am (slowly) learning this electronics thing, but before I build a
> circuit, I want to run a "debounced" power supply past people who know more.
>
> The application is a wireless remote control for a camera system. The
> control will transmit a start bit, and 8 data bits using manchester
> encoding over a 418MHz wireless link. I have the firmware working, and a
> working prototype circuit built in a breadboard. The core component is a
> PIC10f206. The intention is to run the system from a 3V lithium coin
> cell. I intended to put a momentary switch on the +3V line as the
> trigger for the remote. When power is applied, the 10F enables the TX,
> waits 1ms for it to settle, then transmits the data at 10kbaud. it then
> powers down the TX, and goes to sleep. The entire "awake time" is less
> than 3ms.
>
> Unfortunately the switch bounces, and I sometimes get garbled
> transmissions to the RX.
>
> I figured I could solve the problem by putting together a "bounce-proof"
> switch. I have attached a schematic.
>
> Using a P-Channel high-side switch, with the gate connected to the +3V
> via a parallel R and C. The momentary switch is connected between the
> gate and ground. The idea is that with the switch open, the resistor
> will bring the cap to equilibrium at +Vcc, thus putting 0Vgs on the
> gate, and switching off the p-channel mosfet. There will be no current
> drain from the battery then because of the high impedance of the mosfet.
>
> When the switch closes, it "discharges" the cap (effectively)
> instantaneously, bringing the gate to -3V relative to the source, and
> causing the mosfet to conduct. It will conduct until the switch
> re-opens, as well as the time it takes for the cap to recharge to above
> the Vth of the mosfet. The "recharge" time will be proportional to the
> resistor, and cap.
>
> I figure I need the mosfet to conduct for 10ms at least. Once the PIC
> goes to sleep I have measured that it consumes about 25uA, so I am happy
> to let it just sit there since I will have about 7000 hours of battery
> life at that level... ;-) All the mosfet needs to do is to conduct
> through the bounces.
>
> My math indicates that with a 3V supply, and a -2V Vth on the mosfet,
> and that an RC time constant is for about 2/3 charge of the cap, and I
> need 1/3 charge to cut-off, that the RC constant should be about 20ms
> for the RC. Using a 1MOhm resistor (to reduce current draw when the
> switch is closed to 3uA), I need a cap of about 20pF.
>
> The questions I have are:
> 1) will this system work reliably (it appears to work at 5V in my
> breadboard now)?
> 2) 20pF seems real small, I am thinking I should allow more current to
> flow, and use a larger cap, but this will have (small) implications on
> battery life. Perhaps I should go with 100K and 0.2uF, allowing 30uA
> current to ground if the switch is held closed (remeber, this is going
> to be jammed in a camera bag somewhere, and could potentially be held
> closed for a long time...).
> 3) Although I can find a number of SMT p-chan mosfets that have a max
> Vth of -2V, I can't seem to find parts that are Thru-home (can put in to
> breadboard...). At worst case I will just solder some leads right on to
> a DOT-23 package or something, but perhaps someone knows where I can
> find a -2Vth p-mosfet that is readily available (digikey hopefully) in a
> t-hole package.
>
> Thanks in advance for any insights/criticisms.
>
> Rolf
>
>
>
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