About printf (Debugging, Serial Output)

This document describes printf (debugging, serial output).

About printf (Debugging, Serial Output)

printf Library

This is the printf() process used within the TWENET library. For more details, please refer to TWENETmuc/printf.

PRINTF for Debugging

The PRINTF() macro in the fsl library provided by NXP is used for debug output and is excluded from compilation during release. The TWENET library has also been adjusted to use PRINTF(), but it doesn’t have all the features of the fsl library.

The PRINTF() macro calls printf_() mentioned above.
Input macros like GETCHAR() are not supported.

With JN518x SDK 2.6.3 and 2.6.4, NewLib and NewLibNano show garbled or missing data in output and do not behave as expected (this is likely a buffering issue. RedLib does not show this problem, but it cannot be used in a mixed C/C++ project). For this reason, the PRINTF() macro has been changed to use the printf_() function within libTWENETmcu/printf.

SDK_DEBUGCONSOLE Definition

The behavior of the PRINTF() in your application will change based on the value of SDK_DEBUGCONSOLE.

Value	Description
0	Output is sent to the debug console (Semihosting). This process is very slow. * To enable this output, you must set `SDK_DEBUGCONSOLE=0` in the libTWENETmcu (Debug build), rebuild the library, and periodically call `_putchar(-1)` (approximately every 16ms) in your application code. (See `SysTick_Handler()` defined in the sample `Samp_bare_MMAC`)
1	This is the setting for a Debug build. `PRINTF` uses the `printf_` function (libTWENETmcu). If `_putchar()` is not redefined, `SERIAL_bTxChar(0, c);` is called. This is also the default setting for Debug builds of TWENETxxx libraries.
2	This is the setting for a Release build. `PRINTF` is excluded from compilation, and `printf_()` outputs nothing. If you want to use `printf_()`, you must redefine `_putchar(int)`. This is also the default setting for Release builds of TWENETxxx libraries.

The _putchar() in the libTWENETmcu library has a weak link specification. The _putchar() you define in your application code will take precedence.
To enable PRINTF() within the TWENETxxx library, you must set the same SDK_DEBUGCOSOLE definition in your application and rebuild the library.

About SWO

TWENETmcu/source includes code for SWO output. While we don’t officially support this feature, here’s a breakdown of its code and functionality:

Code related to SWO is defined with -DDEBUG and -DSERIAL_PORT_TYPE=1. When these are set, the PRINTF() output is adjusted to be sent to the SWO ITM (main/retarget_itm.c, main/retarget_putchar.c).
The SWO port is set to PIO14.
Debugging often fails to start when SWO is enabled. Keeping the ISP pin (PIO5) in a LOW state until just before the debugger starts can sometimes resolve this.