How to transmit ASCII over RS-232?

Question

i want to display the voltage level through a Potentiometer over the serial comm line(RS-232) in pure ASCII. I am trying to do this using MC9S08AW60 on the Board-DEMO9S08AW60E. So, if the potentiometer is at 50% position, on Hterm it should show as following:

050%

Now for this(6 bytes) i need to be able to convert decimal to ASCII and transmit the same over RS232. I cant seem to find the logic behind conversion of decimal to ASCII.

questions:

1. logic for conversion of decimal to ASCII?

2. any better ideas?

3. if this question is outside the scope of this forum i am more than willing to change it to the appropriate forum.

Consider the following code:

if(TPM1SC_TOF == 1)
   {

     bob = ADC1RL;               
     dummy = (bob*100)/255;
     if((SCI1S1 & (1<<7)))
      { 
       SCI1D = dummy;  // at this point i would like to send the percentage of the 
                       //potentiometer value!
      }
     TPM1SC_TOF = 0; 
   }

Please take note of the attached block diagram!!

Answer 1

So I dug a little bit for you. Take a look at Freescale's "HCS08 Peripheral Module Quick Reference"which is linked from the product page. There is a section titled "Using the Serial Communications Interface (SCI) for the HCS08 Family Microcontrollers." And Subsection 2 of that section is titled "Code Example and Explanation." Follow the bouncing ball and that should get you going. The Dev board already has the circuits needed to go from TTL UART to RS232. There's also code examples in there for the ADC. You'll then just need to manage converting your ADC readings into ASCII using "the usual methods" like, for example, sprintf from stdio.h.

Half the battle with acclimating yourself to a new controller/processor is figuring out where the examples and support ecosystem reside. Hope this helps. P.S. The code examples zip file referenced in the Quick Reference is here (not the easiest thing to find).

Answer 2

Here's part of a minimalistic printf implementation I've been using. It was originally written by Georges Menie of www.menie.org, according to the GPL notice at the top of the file. I have been modifying the implementation as a whole, and haven't really been keeping track of the changes, though, so you probably should use the original which seems to be available at http://www.menie.org/georges/embedded/#printf

The function listed below converts an int to its ascii string. You can try to follow the code and strip out the parts which you don't need. There are calls to prints in the function, which deals with padding and handling the width of the string. It also does the actual sending of chars via putchar. I haven't included that function here to reduce clutter. You should implement your own function to to do that here, which basically would send out each of the chars via your serial/rs232/uart peripheral until you encounter \0.

static int printi(char **out, int i, int b, int sg, int width, int pad, int letbase)
{
    char print_buf[PRINT_BUF_LEN];
    register char *s;
    register int t, neg = 0, pc = 0;
    register unsigned int u = i;

    if (i == 0) {
        print_buf[0] = '0';
        print_buf[1] = '\0';
        return prints (out, print_buf, width, pad);
    }

    if (sg && b == 10 && i < 0) {
        neg = 1;
        u = -i;
    }

    s = print_buf + PRINT_BUF_LEN-1;
    *s = '\0';

    while (u) {
        t = u % b;
        if( t >= 10 )
            t += letbase - '0' - 10;
        *--s = t + '0';
        u /= b;
    }

    if (neg) {
        if( width && (pad & PAD_ZERO) ) {
            printchar (out, '-');
            ++pc;
            --width;
        }
        else {
            *--s = '-';
        }
    return pc + prints (out, s, width, pad);
}

Answer 3

While you can probably do this with a printf-family function on that platform, it's informative to consider how you might write you own implementation. For example:

1. Find a power of ten larger than the maximum value - in this case your desire for leading zeros will simplify things.

2. Divide by decreasing powers of ten, to produce each output digit, retaining the remainder to feed the next calculation.

3. Convert each digit from a value to its ascii representation. For decimal, this is easy as you just add the ascii code of the character '0' (which is 0x30or 48 in decimal) to the decimal value. (For hex digits, you have to handle 0-9 and a-f in separate clauses)

4. Write the resulting bytes to the serial port, typically waiting until the UART transmit buffer is empty before writing each one.

The starting point code you've added seems to contemplate doing the conversion inside the transmit buffer check; that won't really work for multi-byte sending. Instead, your outer loop doing the ADC sampling would have the decimal-to-ascii conversion loop inside it, and inside that at the point where it's generated a single character would be a while looping waiting for the serial port to be ready for a character. (That is, assuming you don't need to do other things in the meantime - if you do, you'll need a software-managed transmit buffer and something such as an interrupt service routine to pull waiting characters out of the buffer and send them each time the serial port becomes able to accept one).