Mystery instability of 16f628a

[Davidmarks]

Having struggled unsuccesfully to get a simple pendulum driver program working I have gone back to basics in order to check out my programming. i have built a basi cbreadboard with a 16f628A It simply has the supply connected (5V) and the 5 lines necessary from a pickit 2. There is an LED with 1K in series hanging on port b.3. Sometime it works sometime it doesn't.
Bringing a finger to within an inch of the pickit2 makes it run. Have I gone mad (again) or have I just done something unspeakably daft ?

[joseph]

add a 4.7k pull up resistor on mclr pin

[Bruce]

Have I gone mad

Quite possibly - but we will hold-off sending the guys with the white coat over until we've seen schematics and code...

[Davidmarks]

Thanks for that Bruce - I should have asked matron to check my post - forgot to attach the code, Just arrived back from the village pub and tried Joseph's 4.7k pull up on mclr and lo and behold... it works ! many thanks for that .
Cheers David

[Davidmarks]

I do not believe this ! After a spot of tinkering and adding to the circuit this am it has suddenly gone unstable again (It only operates when finger approaches or touches PIC) stripped circuit down to basic (schematic attached) and it still doesn't work. By the time i have finished it would be quicker to build the thing with valves (tubes) I used to understand them ! The code is as follows:
[@ device pic16f628a,INTRC_OSC_NOCLKOUT
define osc 4
begin:
low porta.0 ' Turn LED on
PAUSE 500 ' Wait 500mS (1/2 second)
high porta.0 ' Turn LED off
PAUSE 500 ' Wait 500mS (1/2 second)
GOTO BEGIN ' Loop continuously
end

[master_olan]

Have you try turning off the analog comparators? if you dont know how just add ADCON1=7 near the top row...