Newbie / Frustration (very both)

[iso9001]

I just setup a low voltage programmer and got some 16F872 (acutally they're 16LF872)... got the programmer (tlvp) working, made a simple led example, programmed it, setup a test board/led deal (5v)and................. nothing happends.

Here is the code:

[davt]

Hi
You must set up the ports properly - PORT and PORT DIRECTION (Tris)
registers. Then your program should then work.
Good luck!
Dave

[iso9001]

But I'm using output_high(port); so shouldn't standard_io take care of that for me ?

That was my understanding, the standard_io (default) did the tris automatically.

[Ttelmah]

[iso9001]

I dont understand whats going on here....

[Neutone]

What kind of signal is going to MCLR?

[iso9001]

um..... none ?

nothing is.... is it supposed to be ?


My programmer runs to mclr... but the led setup i have nothing goes to mclr/vpp... I thought that was for programming only...

[PCM programmer]

I saw your post last night, but I didn't answer it because I have
no experience with the TLVP programmer.

But I had a hunch.... I said to myself, "does he even have
a crystal hooked up to his PIC ?"

In addition to Neutone's remark about MCLR, you should
look at this schematic to see how a crystal is connected to
the PIC. It has to go on the OSC1 and OSC2 pins.
You need the capacitors also. The #use delay statement
value must match the value of the crystal frequency.
See this schematic. Note that the pin numbers are
different, because the schematic shows a 40 pin PIC.
But you can go by the pin labels, and compare that
to your 16LF872.

http://www.melabs.com/downloads/labx2sch.pdf

In addition to that, you very likely need a resistor in
series with your LED. Use a value from 220 to 470 ohms.

[iso9001]

Ok.... what if i want to do this w/o a clock.... Cant I use an internal timer or somthing or do i NEED an external clock ???

What is SUPPOSED to be sent to mclr? (i dont want or need a reset)

I just got back from radioshack... No Clock/Crystals :( .... Is there a store that is probally local that would have these ??

Also, i'm using a 5V led that i know works.


edit: Um... in FPP it says "Config: ####" should somthing meaningful be in there or does it retrieve it from my hex file? ...And if I am supposed to put somthing in there, i'm guessing its a hex value, but how do i figure what to put ?

[Ttelmah]

[iso9001]

I see,

So.... I need to go back to radioshack and get a 47k pull up resistor. Then 5V goes to the resistor and then that goes to mclr... this is NEEDED ?? I've never seen anyone's simple led diagrams w/ this... then again, those have all been for 16f84s

So for the clock I need to go find a crystal (or i guess use RC fuse... is there a forumla to figure out what X resistor and Y cap with make for Mhz ?)

Anyhow, this is what I cam up with now... will this one work ???


Oh, and your right, there should be a resistor on that led... what resistor can i put on there that wont cut the brightness down too much but will stop it from burning the pin ?

Thanks Again,


edit: also, I'm trying to find 22pF caps... its pico right, not mirco (uF)... It looks like radioshack MIGHT have a set of 80 mixed caps that have 22pF in there, but its like $8.00! They do have 100pF caps, will those work? (prolly not, but hey, might as well ask)

[rwyoung]

Radio Shack is nearly worthless for getting parts.

Since you have obivously have some kind of Internet access, go to www.digikey.com and www.mouser.com (there are others but these will get you a long way). Prowl around and find an email or phone number to call and order a catalog. The catalogs are available on-line but I find it is sometimes easier to sift through the printed version first.

You will have minimum order issues and shipping costs but weigh those against the cost of your time and frustration when you don't have the parts you want to try an idea. Also, if the part is cheap order 2 or 10 or 100, never just one.

Also, go to http://www.microchip.com/1010/suppdoc/refernce/midrange/index.htm and download a copy of the midrange reference manual. Good stuff in there.

Back to your circuit,

1) You need MCLR- pulled up to Vdd (+5V) Use a resistor between 10K and 47K. Radio Shack does have these (1/4W carbon film, 5% is OK)

2) You need some kind of clock source. You can tell the PIC to use its internal clock generator . This is done with the FUSES statement if you are using the CCS compiler. EXTRC is the keyword for external RC. The resistor should be kept between 3K and 100K. It is between Vdd and the OSC1 pin. Then from the OSC1 pin to ground you add a small capacitor. The minimum value of capacitor is 20pF and Microchip recommends you pick something a bit higher. Now to pick the values you can refer to the charts in the DC and AC Characteristics Graphs and Tables portion of the datasheet. Only problem is that the PIC16F87xA data sheet (at least the copy I had handy) doesn't have that appendix filled out yet. As a fall-back position I'd refer to the previous revision's data sheet, the PIC16F87x. There I find that if my Vdd is 5V and I'm at 25C I can expect to have Fosc run at 3.5MHz if I pick 20pF and 3.3k. You can put an oscilloscope on OSC2 to see Fosc / 4 (don't have an oscilloscope, then save your money or try to borrow one). You will NEVER get the kind of frequency stability that a crystal, crystal oscillator or even ceramic resinator provides using an external RC. But to blink a LED, the EXTRC choice is just fine.

3) You need a current limiting device for your LED. The simplest is a resistor. In general, the average RED led has a voltage drop of about 2.1V. And the average RED led has a maximum current limit of 20mA. But you don't need 20mA through the LED to make it nice and bright, most look fine on the bench with 8mA to 10mA. To pick your resistor the formula is R = (Vdd - Vled) / Iled. Since you are probably going to use a 5% resistor and just admire the LED with your eyeballs we don't need to get fancy and have 3 decimal points of precision here. Assume Vled = 2V (I know I said 2.1V earlier but its easier to do in your head if you use 2V) and Vdd=5V. If you want 8mA then you need (5-2)/.008 = 375 Ohm. The closest 5% value is 360 Ohm. However if your only source of parts is Radio Shack you are probably stuck with 330 or 390 Ohm or maybe 470 Ohm. Good enough.

If you get some better LEDs than the crappy surplus junk Radio Shack sells then you can get a nice bright light with less current. Try playing with values of 470 or 1K. Also, if you are going to drive a lot of LEDs from a single chip without multiplexing then you need to make sure you aren't asking the chip to sink or source more total current than the maximum allowed for the IC. You just have one LED so no worries.

4) If you can borrow a known working programmer then you can eliminate several unknowns. That will let you test your circuit and code instead of the programmer, circuit and code.

The last thing I can say is to save your pennies and keep your eyes open for good deals on development tools like programmers, comilers and test equipment. Even working as a hobby, your time and sanity are worth much more than saving $49.95 to buy the tool that does what you need.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[rwyoung]

I just looked at the Microchip web site and they have an updated copy of the PIC16F877A datasheet. It does have the frequency vs R vs C vs temp graphds included. Somewhat different from those in the non-A datasheet so look at the new ones.

Don't try 3.3K and 20pF. Start with 10K and 100pF to run around 1MHz Fosc. You can try 5.1K and 20pF to get close to 4MHz but don't expect it to be reliable if your Vdd goes below 4.5V or the temperature shifts more than a few degrees away from 25C.

Time to print a new datasheet and transfer all my notes to the new pages!
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[PCM programmer]

He's not using the "A" part. See this quote from his previous post:

[Guest]

oh...

Yea, confusing sorry.


So that pic(ture) I have there will work now ? Cause I found out that NO ONE in 100mi of me has clocks... So i'm gonna make a RC deal today and have some ordered in.

(btw: whats the diff between a crystal clock and an ocillator clock?)