2.5.8 How Do I Use Printf to Send Text to a Peripheral?
The printf function does two things: it formats text based on the format string and
placeholders you specify, and sends (prints) this formatted text to a destination (or
stream); see Appendix A. Library Functions. The printf function performs all the formatting; then it calls a helper function, called putch, to send each byte of the formatted
text. By customizing the putch function you can have printf send data to any peripheral or location (see Section 4.12 “Mixing C and Assembly Code”). You can choose the
printf output go to an LCD, SPI module or USART, for example.
A stub for the putch function can be found in the compiler’s sources directory. Copy
it into your project then modify it to send the single byte parameter passed to it to the
required destination. Before you can use printf, peripherals that you use will need to
be initialized in the usual way. Here is an example of putch for a USART on a mid-range
device.
void putch(char data) {
while( ! TXIF) // check buffer
continue; // wait till ready
TXREG = data; // send data
}
You can get printf to send to one of several destinations by using a global variable
to indicate your choice. Have the putch function send the byte to one of several destinations based on the contents of this variable.
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