Matrix _16f877 need help, please

[Archangel]

hi, Joe and Steve,
thanks for your concern. I tried testing but no thing happen. is it possible the two capacitors of the osc-xtal is the main problem.

I saw in the data sheet, for the Hs (20Mhz) xtal osc, the capacitors are range 15-33pF. but i use only 10uF! (table 14-2 page 144 of the data sheet).

µ is Way bigger than pf Way too big !10µF will oscillate in kilohertz, not mhz.

[MINHLE]

thanks, I should replace them tomorrow.

[Dave]

MINHLE, What exactly is the question? Are you concerned that the voltage across the led is too low? What is the value of the resistor in series with the led? Do you have any resistor in series with the led at all? If there is no resistor in series with the led then the current is too great for the PIC and the voltage is being dropped too low for the oscillator to operate... What is the voltage across the 2 PIC pins you are using to drive the led? You should start out with maybe a 680 Ohm resistor in series with the led and see if that doesnt cure the problem...

Dave Purola,
N8NTA

[MINHLE]

Hi Dave,


thanks for your concern,

the voltage drop across the LED is an important issue here. if the voltage different across the LED is '0" then the LEd cannot be lighted up. a resistor in serial with the LED is to protect the LED when the high pulse current flowing through the LEd aND TO PROTECT THE PIC FROM HIGH IN PUT CURENT . so we do have to consider the voltage drop on the resistor.

in this case the positive pin of the LED is 1.65V, then the negative pin of the LED has to be '0' as set up. however the situation different, both side of the LED is 1.65 so it means that either open circuit (no current flowS through the LED) , the bad PIC or the program not well set up( i.e conf, ...).

I wish getting some suggestion , ideal from you,

thank agian

[aratti]

Since you didn't use a limiting resistor, you have fried the pic!
Replace it and put a 330 ohms resistor in series to every leds. Then use Joe snippet, given in post # 2 and you will see that leds will switch on and off without burning another pic.
You will always read 1.6 - 1.7 V across a led, unless you have shorted one!

Al.

[MINHLE]

hello Arrati,

yes I have 200 ohms resistors connected in serial with each input of the PIC to limit the current into the pic.

I guest some thing wrong, because when I use OHM meter to test the LEDs. at the two pins on the socket (removing the PIC out of the socket) every thing is ok! so the circuit is not open.

I try to figure out

thanks for your comment.

Minh le

[MINHLE]

hi every boDy,

I removed the two capacitors of the X-TAL OSCILLATOR then it works. i am sorry for long time not to post the reply