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rnome
Joined: 18 Dec 2006
Posts: 20
Location: europe (Turkey)
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| Digital output voltage is too low |
 Posted: Wed May 11, 2016 2:44 pm |
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Hi all,
I'm developing a led driving unit with 4 different modes (on,off,async,sync). I prepared both software and hardware but it seems there is something wrong in the code. I suspect the software because real time test and simulation gives the same failure output.
at first here is the code
| Code: |
#include <16F628A.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)
#use fast_io(A)
#use fast_io(B)
boolean pressed=false;
boolean toggle=false;
int mode=0;
int counter=0;
#int_TIMER2
void TIMER2_isr(){
++counter;
if(counter==50){ //250ms
counter=0;
switch(mode){
case 0: //all off
output_low(PIN_B1);
output_low(PIN_B2);
output_a(1);
break;
case 1: //all on
output_high(PIN_B1);
output_high(PIN_B2);
output_a(2);
break;
case 2: //flash syncron
output_toggle(PIN_B1);
output_toggle(PIN_B2);
output_a(4);
break;
case 3: //flash asyncron
output_bit(PIN_B1,toggle);
if(toggle)
toggle=false;
else
toggle=true;
output_bit(PIN_B2,toggle);
output_a(8);
break;
}
}
}
void main(){
output_b(0x00);
output_a(0x00);
set_tris_b(0x01);
set_tris_a(0x00);
setup_timer_2(T2_DIV_BY_16,0x82,12); //4*16*130*12/20Mhz=4.99ms
enable_interrupts(int_TIMER2);
enable_interrupts(GLOBAL);
for(;;){
if(input(pin_b0)==1){
if(!pressed){
pressed=true;
mode++;
if(mode==4)
mode=0;
}
}
else{
pressed=false;
}
}
}
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There are 2 LEDs connected PortB_1 and PortB_2 and each time I press a button which is connected to PortB_0, light mode of the 2 leds will change. Modes are : both LEDs off, both LEDs on, LEDs flicker synchronously, LEDs flicker asynchronously.
No matter what I do, LED1 is always off and LED2 starts working only on last mode but voltage output of the PortB_2 is around 1v which is quite odd. I suspect of fuse selections but there are the ones that I use before.
So anyone has an idea, is extremely welcome 
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live fast, never rest and do your best |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed May 11, 2016 4:02 pm |
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Post your schematic.
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rnome
Joined: 18 Dec 2006
Posts: 20
Location: europe (Turkey)
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| Posted: Thu May 12, 2016 12:45 pm |
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Hi PCM programmer,
you can find my schematic as below.
Those 4 set of LEDs which connected at port A are used as mode indicator.
ps: I used pull-up resistor at MCLR on my real circuit.
_________________
live fast, never rest and do your best
Last edited by rnome on Thu May 12, 2016 1:22 pm; edited 1 time in total |
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ezflyr
Joined: 25 Oct 2010
Posts: 1019
Location: Tewksbury, MA
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| Posted: Thu May 12, 2016 1:20 pm |
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Hi,
Well, for such a simple circuit, you've made just about every mistake that can be made 
To wit:
1. You need current limiting resistors in series with the base of each transistor. Something like 10K should be fine.
2. Your current limiting resistors in series with LED1 and LED2 are too large. Something like 500 ohms is OK.
3. You have no current limiting resistors in series with LED3, LED4, LED5, and LED6. Again, 500 ohms is OK.
4. You have no pull-up on MCLR. A 10K resistor is probably good here.
5. It's much more common to put a 10K pull-up on your switch input, and connect the free end of the switch to Gnd. Thus, pushing the switch pulls the switch input to Gnd, while releasing it causes the input to go high via the pull-up.
I suspect that there is no hardware here at all, right? IOW, this is a pure simulation?
_________________
John
If it's worth doing, it's worth doing in real hardware!
Last edited by ezflyr on Fri May 13, 2016 6:28 am; edited 1 time in total |
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rnome
Joined: 18 Dec 2006
Posts: 20
Location: europe (Turkey)
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| Posted: Thu May 12, 2016 1:31 pm |
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Hi ezflyr,
Thanks for your input. You are right this one is just for simulation so I didnt deal with current limiting resistors on LEDs and NPN base but in real circuit they exist with reasonable values ;)
So I think my chip might be a defective one. I will try with another one and share the outputs.
_________________
live fast, never rest and do your best |
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temtronic
Joined: 01 Jul 2010
Posts: 9215
Location: Greensville,Ontario
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| Posted: Thu May 12, 2016 1:54 pm |
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re: LEDs
I'm curious as to why you decided to 'direct drive' 4 of the LEDs and not the other 2 ? The PIC is 'happy' driving LEDs, just use a 500r in series as posted above, there is no need for the NPN transistors and base resistors.
Also if you can live with a 4MHz clock, use the internal one...saves 3 components, some wiring and is reasonably accurate.
Jay |
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rnome
Joined: 18 Dec 2006
Posts: 20
Location: europe (Turkey)
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| Posted: Thu May 12, 2016 2:06 pm |
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Those 2 individual LEDs will be driven by higher voltage (around 12v) thats why I separated them.
Yeah you are right, maybe I can use internal clock. 4Mhz is good enough if I can get 5ms timer interrupts. I will check timer parameters.
_________________
live fast, never rest and do your best |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19477
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| Posted: Thu May 12, 2016 2:15 pm |
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Funnily enough this is a case where the simulation 'knows' that LED's have a Vf, and is doing the correct thing, and stopping the PIC working when it is massively overloaded....
Also the two outputs driving the transistor bases are being clamped to just over 0.6v. A real PIC might well start releasing magic smoke at this point. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu May 12, 2016 4:18 pm |
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Here's a page that discusses the transistor base resistor size in a fair
amount of detail. It's a good tutorial page:
http://www.ermicro.com/blog/?p=423 |
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