autobaud.doc

Automatic Baud Rate Detection for the 80C51

Automatic Baud Rate Detection for the 80C51                             AN447

This note documents a method to automatically establish the correct baud rate 
for serial communications in many 80C51 family applications. The first 
character received after a program is started is used to measure the baud rate 
empirically. 

This can eliminate the need to have setup switches whose settings are 
difficult to remember and all of the other headaches associated with 
applications that use multiple baud rates. One might assume that a reliable 
method of accomplishing this might be impossible without severely limiting the 
characters that could be recognized. The problem is in finding a timing 
interval that can be measured in a large number of possible characters under a 
wide variety of conditions. 

Measuring a single bit time would be the obvious way to quickly determine what 
baud rate is being received. However, many ASCII characters don't have an 
example of a single bit time in the RS-232 pattern. For most characters, the 
length of the entire transmission from the start bit to the last "visible" 
transition will fall within certain ranges as long as some reasonable 
assumptions can be made about the possible baud rates (i.e. that they are 
standard baud rates). Moreover, many systems now use 8 data bits and no parity 
for ASCII transmissions. In this format, normal ASCII characters will never 
have the MSB set and since UARTs send data LSB first/MSB last, the program 
would always be able to "see" the beginning of the stop bit. 

The following baud rate detection routine waits for a start bit (falling edge) 
on the serial input pin and then starts timer 0. At every subsequent rising 
edge of the serial data, the timer value is captured and saved. When the timer 
overflows, the last captured value will indicate the duration of the serial 
character from the start bit to the last 0 to 1 transition (hopefully the stop 
bit). 

The table CmpTable contains the maximum timer measurement that is accepted for 
each baud rate. These values were picked such that a timed interval of only 4 
data bit times (plus the start bit time) will still produce the correct baud 
rate. 

There is an assumption in this method that anyone using it needs to be aware 
of. That is, that this technique depends on only one character being received 
during the sampling window, which has to be at least as long as a typical 
character at the slowest baud rate that can be accepted. Essentially this 
means that the data must normally come from someone typing at a keyboard. 

On our PCs, we were not able to fool the program by typing two characters in 
quick succession. The PC function keys did present a problem because they send 
two characters in a tight sequence, and fooled the program into detecting the 
wrong baud rate. In the example program, which is designed for a 12 MHz clock, 
the total sample interval is about 65 milliseconds, or about twice the 
duration of an RS-232 character sent at 300 baud. 

If parity is used, a possibility of a baud rate determination error happens 
when the four MSBs and the parity bit of the character received are all ones. 
This can happen for the lower case letters "p" through "z", plus curly 
brackets, vertical bar (|), tilde (~), and "delete", depending on whether the 
system uses odd or even parity. Note that the usual prompt characters that a 
user would type to get a system's attention (e.g. space, carriage return, and 
escape) are NOT subject to this limitation. 

Note that, because of the way this program works, the first input character 
that is used to detect the baud rate is lost since the UART cannot be set to 
the correct baud rate until after the first character has been timed. Also, 
most "real" programs using this technique would want to repeat the baud rate 
detection process if framing errors are detected at the UART during normal 
operation. 

To calculate CmpTable values for other oscillator frequencies and baud rates, 
use the following equation: 


                 Osc (MHz)        5
Table entry  =  -----------  *  ----
                 Baud Rate       12


Remember that the table entry is a two byte value, so the result of the above 
must be split into upper and lower bytes (easy if you have a hexadecimal 
calculator). It may also be possible to get the assembler to do all of the 
calculations for you. 


The above equation was derived as follows:


  maximum          minimum recognition time
timer value   =  --------------------------
(table entry)       machine cycle time


    minimum           bits-to-recognize
recognition time  =  -------------------   *   byte time
                         #-of-bits 


Note: '#-of-bits' (the number of "visible" bits) is 9, and bits-to-recognize 
(the minimum # of bits to recognize) is 5 for 8-N-1 communication. 


                   1
byte time  =  -----------   *   #-of-bits
               baud rate


 machine        Osc frequency
cycle time  =  ---------------
                     12