Page 424 - Programming Microcontrollers in C

Serial Input/Output 409

that can be easily changed if the chip itself is changed. The defines
and enums set at the beginning of Listing 8-7 are from the top of the
serial1.c program. The values established by these devices are
used in the functions of Listing 8-7. These #defines and enums
would not be necessary if the compiler worked as expected. Generally,
if you want to wait until it is safe to send a character out the serial
port, the code that you might use is
while(U1SR.TDRY ==0)
; /* wait here until TDRY is 1 */
This code will cause the computer to sit in a loop while the TDRY bit
found in U1SR is 0. Unfortunately, the code created by the compiler
failed to reload the value of the bit inside the loop, so the computer
went into an infinite loop when it executed this code.
There are always ways around such problems and with C it is not
usually necessary to jump to assembly language when such a problem
is found. Notice the code below. Rather than the argument shown
above, the argument was converted to a simple bit-wise AND, which
did compile correctly. Remember, the addition of a #define or an
enum in your code does not add or subtract from the executable
code in your program.
The first two functions shown below are put() and
getchar(). The function put() sends a character to the serial
port number 1 on the board. It works with the function putchar(),
which works exactly the same as the putchar() function that you
are used to using in your programs. Here putchar() sends a
character to the serial port rather than to the device stdout. The
function getchar() receives a character from the serial port 1. In
C, when a ‘\n’ character is sent to the output the program executes
a carriage return and a line feed. Therefore, to make getchar()
here the same, you will see in the next group of functions that the
data received by getchar() is tested. If it is a ‘\n’ character, two
characters, ‘\n’ and ‘\r’, are sent to the serial port. Otherwise,
the character passed to the function is sent to the serial port. This
operation mimics the operation of the normal getchar(), but it
also requires a special function to output the character to the serial
port. The function put(BYTE x) is that function. Since the function
put() is never to be used outside of this file, it is designated as
static.