16f877 rs232 problem and some Matlab

[strumpfm]

Hi all- I know this has been brought up a million times before, but it seems like there's one small barrier that everyone needs to get past, and I'm stuck at one.

I'm trying to eventually collect data in matlab to plot off my PIC, but for the time being I would just like to see something in HyperTerm or something. I'm using an 877 with the Tx on pin B7 (pin 2 of serial port) and Rx on B6 (pin 3 of serial port) connected to the computer with a 22k resistor, pin 5 of the serial port grounded, using the internal clock, not an external crystal, and attempting to use software USART. Here's the code that I've been trying:

#include <16F877.h>
#include <String.h>
#include <Math.h>
//#device PIC16F877
//#device adc=10
#FUSES HS,NOWDT,NOPROTECT,NOPUT,BROWNOUT,NOLVP,NOCPD,WRT
#USE DELAY(CLOCK=10000000)
#USE RS232 (BAUD=9600, xmit=PIN_B7, rcv=PIN_B6, PARITY=N, BITS=8,ERRORS, invert)
#byte port_b = 6
#byte port_c = 7
#byte port_d = 8


void output_int(int var);
void cycle();
void clear_out();


void main()
{
int temp;
int i=0;
int toggle=0;
set_tris_b(0x00); //set b as outputs
set_tris_d(0x00); //set d as outputs
delay_ms(50);
do{

for (i=0; i<1000; i++) {

printf( "Hello!" );

} //end for

}while(1);
}

I tried using HyperTerminal set up with 9600 baud, 8 data bits, no parity, stop bits of 1, and hardware flow control. The com port will open, but nothing comes out on the screen. I also tried writing a Matlab function to read in data from the serial port that looks like this:

ser_obj=serial('COM2','baudrate',9600);
fopen(ser_obj);
rctime=fscanf(ser_obj,'%s'); % receive sensor data
fclose(ser_obj);
y=rctime;

I don't know if any of you know Matlab serial interfacing well, but I just keep getting a message saying "Warning: The input buffer was filled before the Terminator was reached."

Am I overlooking something really basic? Any little bits of guidance would be greatly appreciated. Thanks,

Mike

[PCM programmer]

[strumpfm]

[Pete Smith]

[TSchultz]

For RS-232 to work reliably the baud rate has to be maintained within +/-3% at both ends. You should be using an external crystal, some of the ceramic resonators are OK as well. Then you need to pick a crystal frequency that minimizes the errors for the baud rate you want to use, a frequency easily divisible by powers of 2 are the best. Check the data sheet and read the section on the USART, you will find most of the details you need in there.

Next you should always use a hadware level shift device for the RS-232 signals, they are supposed to be bipolar 12V signals. Although many devices will accept unipolar low level signals you are not guaranteed of this. Use a MAX232 or one of the many equivalent parts and things will be much more reliable.

Also use the hardware USART on the PIC16F877, it will be much more reilable than the software one you are currently using. Most of the problems people encounter with the RS-232 is not understanding the interface. Using the hardware USART and the hardware level shifting takes most of the guesse work out.

Start of with a simple "Hello world" before you start sending data. If you can't send a simple string then you won't be able to send data.

If you search this forum you should find a great deal of information on the subject.

[rwyoung]

[PCM programmer]

[strumpfm]

Thanks for all of your help, I certainly know more now. After realizing that using a resistor for driving the clock isn't accurate enough, I was able to get an XTAL-20MHz crystal. I've been coming up dry when searching for info on these crystals, though. Do these crystals have a polarity? I've seen designs that have both ends of a crystal tied to ground, and the inputs in OSC1 and OSC2. Most ranges that I've seen for capacitance were between 15 and 30 pf. I had some 22pF capacitors, so I connected them up, but wasn't able to get oscillation. Is that how you are supposed to connect up the oscillator? One should be able to hook up a scope and view the oscillation at OSC1, right? Thanks again,

Mike

[PCM programmer]

Look at the following schematic. Zoom in on the crystal circuit
on the left side of the 16F877. Do you have your board wired
in the same way ?
http://www.melabs.com/downloads/labx2sch.pdf

Also, since you're using a 20 MHz crystal, you need to use HS mode,
and you need to set the #use delay value to 20 MHz. Example:

#include <16F877.H>
#fuses HS, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 20000000)
#use rs232(baud = 9600, xmit=PIN_C6, rcv = PIN_C7, ERRORS)

[strumpfm]

[strumpfm]

I think that I'm confused about how the crystal operates. I had a basic led test working previously with just a resistor for the clock, but with the crystal set up as in the previous .pdf file, I'm not seeming to get any clock, so none of my program is working. Here is the code that i'm running, but I don't think the code really effects clock generation. Do I need to specify any bits set to use a crystal instead of a resistor?

[PCM programmer]

That isn't really a simple LED test. The following code is a simple
LED test.

Before, you said that you found a 20 MHz crystal, and some 22 pf
capacitors. But the code you posted is setup for a 4 MHz crystal.
Do you know why ?

If you have a 20 MHz crystal, please use the following code, verbatim.

[strumpfm]

[PCM programmer]

1. In your schematic, I don't see any connection to the MCLR pin.
That pin should be connected to VDD. In some cases a resistor
to VDD is recommended. What are you using as a programmer ?

2. Your schematic shows a battery as the power supply. What's the
voltage ?

If you're running at less than 5 volts, there are other issues
that become important. For example, say you're running
at 3.3 volts. Then you can't enable the Brownout feature.
That's because it triggers a reset if the voltage falls below
approximately 4.0 volts.

Also, there are limitations on the oscillator frequency, based on
the Vdd voltage. Vdd must be at least 4.5v for 20 MHz operation.

During these tests, you should be using a +5v regulated power
supply. It would make things a lot easier.

[adrian]

If you have wired it as per your drawing, your LED is in back to front