Electronics > Microcontrollers

PIC16F690 - Output Pin Rippling at High Level

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Mechatrommer:
i programmed 2 output pins of the mentioned pic, pin 1 is (yellow trace below) to output some pwm signal, and the other one, pin 2 (blue trace) to be high at some point to act as Vref for adc's reading using trimpots, and switches (normally grounded) all of them using 10K ohm current limiting resistors. i dont think current draw will be too much from my calculation/estimation. the pic Vdd (3.3V) got smd capacitor nearby as V stabilizer, but unknown capacitance. you can see in the picture, the pin 2 (blue) is rippling but pin 1 (yellow) is ok. i dont think its because the inadequate capacitance of the unknown capacitor since pin 1 is ok. is there any other possible cause for this rippling?

deephaven:
Pin 1 looks a bit low in amplitude, is it being loaded?

Try touching another capacitor between VDD and VSS to see if that has any effect on the ripple.

Mechatrommer:

--- Quote from: deephaven on October 05, 2011, 12:01:26 pm ---Pin 1 looks a bit low in amplitude, is it being loaded?
Try touching another capacitor between VDD and VSS to see if that has any effect on the ripple.

--- End quote ---
pin 1 is connected to a transistor's base driving something. in between the transistor and pin 1 is a led and 2K ohm resistor. i was probing between the led and resistor (ie the led's lead (anode) closest to pic). so resistor is between the probe and pin 1, maybe a voltage division going on there, but i'm not sure, bugging me a bit though, but not as much as the pin 2. its quite hard touching with finger in that area, the pic and cap and ground plane below it is very small area, outside the pic perimeter are just another traces connecting to other pic's pins. and both my hand are occupied while trying to get this signal (by button press and holding to something). i'll see what i can do.

i dont think (but i'm not sure either) that my poor board layout (pin signal running next to power supply line, poor grounding knowledge, poor diy etched pcb etc) cause this since this is just low speed circuit. pin 2 is probed directly (no component in between) but there's diy via there (soldered strip of wire from top to bottom through diy drilled hole :P)

amspire:
If it was a supply decoupling problem, you would see problems on both traces.

Above Vdd-0.7v, the pullup is very weak, and at 3.3V out, pin2 would probably have very high impedance. So when you attach a probe to it, the probe acts as an antenna.  If you add a 10K resistor (or 100K) from pin 2 to Vdd or Vss, the waveform will probably look a lot better.

Richard

Mechatrommer:

--- Quote from: amspire on October 05, 2011, 01:44:36 pm ---If it was a supply decoupling problem, you would see problems on both traces.

--- End quote ---
it is power supply coupling problem, i've probed directly into pin 1, its the same rippling pwm. power supply is using 12V small 23A battery, stepped down to 3.3V using LTHT7333 ldo-vreg. the 3.3V supply is rippling. i'm working on it. i think its lacking the input coupler. its late will continue tomorrow. thanks guys for the thought. the LTHT7333 max current is 150mA, do you think this is enough for the pic general purpose?

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