PIC16F84A EEPROM problems

[The Master]

righty. now im at home i can give you th full code. i made an new test program and it still doesnt work

trisa=%00000000
trisb=%00000000

t1 var byte

high portb.0
pause 100
low portb.0
pause 100


high portb.3
pause 100
low portb.3
pause 100

read 0,t1

portb=t1

end

it has 8 LEDs connected to portb. 2 of them flash on startup to prove the chip is working then it should read a value from EEPROM[0] and display it using the LEDs.

i set the EEPROM values in the programmer software before programming. if i read everything back from the chip then i can see it definately has set the EEPROM.

no matter what value i put in the EEPROM the LEDs always tell me the value is %11111110

[skimask]

Instead of END, try STOP, or a read/portb/goto loop, see what happens.
Do you have anything across Vss & Vdd?

[The Master]

Instead of END, try STOP, or a read/portb/goto loop, see what happens.
Do you have anything across Vss & Vdd?

i tried the loop

loop:
read 0,t1
portb=t1
goto loop

still same result.

what might i have accross vss and vdd? do you mean power or components? i have power because the LEDs work. i also have a capacitor before the regulator or the chip wont work

[skimask]

what might i have accross vss and vdd? do you mean power or components? i have power because the LEDs work. i also have a capacitor before the regulator or the chip wont work

How about a little something, like a .1uf across Vdd and Vss on the PIC itself?

[The Master]

How about a little something, like a .1uf across Vdd and Vss on the PIC itself?

hmm, do i really need that? the origional diagram i had a few years back shows 1 there but i tried removing it and it didnt appear to affect anything.

ill try and find one. its not something i have laying around :P

[mister_e]

Try..

        @   __CONFIG _XT_OSC & _WDT_OFF & _CP_OFF
        
        PORTB = 0
        TRISB = 0
        
        DATA 0,1,2,4,8,16,32,64,128
        
        Addr var byte
        
        for Addr=0 to 8
            read Addr, PORTB
            PAUSE 500
            NEXT
            
        STOP

and make sure you have MCLR pin tied to VCC via a resistor. Cap always have to be there. 0.1, 0.22, 0.47 or whatever else in that range.

[The Master]

ok, i copied that exactly. it wouldnt compile because of the top line so i removed it and set the fuses in the programmer software. i borrowed one of those caps from another circuit and its still not working. im getting the exact same result

Edit: MCLR is connected to vdd with a 10K resistor

[skimask]

hmm, do i really need that? the origional diagram i had a few years back shows 1 there but i tried removing it and it didnt appear to affect anything.

ill try and find one. its not something i have laying around :P

That was then, this is now... Just bad design practice NOT to use one, I don't think you'll find anybody who will dispute that fact. Just because it worked once, doesn't mean it'll work like that every time...however, conversely, you'll have a hard time finding a PIC that won't run because one is installed (vs. one without a cap installed).