Retarget IO (retarget-io)

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Overview

A utility library to retarget the standard input/output (STDIO) messages to a UART port. With this library, you can directly print messages on a UART terminal using printf(). You can specify the TX pin, RX pin, and the baud rate through the cy_retarget_io_init() function. The UART HAL object is externally accessible so that you can use it with other UART HAL functions.

NOTE: The standard library is not standard in how it treats an I/O stream. Some implement a data buffer by default. The buffer is not flushed until it is full. In that case it may appear that your I/O is not working. You should be aware of how the library buffers data, and you should identify a buffering strategy and buffer size for a specified stream. If you supply a buffer, it must exist until the stream is closed. The following line of code disables the buffer for the standard library that accompanies the GCC compiler:

setvbuf( stdin, NULL, _IONBF, 0 );

NOTE: If the application is built using newlib-nano, by default, floating point format strings (f) are not supported. To enable this support, you must add -u _printf_float to the linker command line.

RTOS Integration

To avoid concurrent access to the UART peripheral in a RTOS environment, the ARM and IAR libraries use mutexes to control access to stdio streams. For Newlib (GCC_ARM), the mutex must be implemented in _write() and can be enabled by adding DEFINES+=CY_RTOS_AWARE to the Makefile. For all libraries, the program must start the RTOS kernel before calling any stdio functions.

Quick Start

  1. Add #include “cy_retarget_io.h”

  2. Call cy_retarget_io_init(CYBSP_DEBUG_UART_TX, CYBSP_DEBUG_UART_RX, CY_RETARGET_IO_BAUDRATE);

    CYBSP_DEBUG_UART_TX and CYBSP_DEBUG_UART_RX pins are defined in the BSP and CY_RETARGET_IO_BAUDRATE is set to 115200. You can use a different baud rate if you prefer.

  3. Start printing using printf()

Enabling Conversion of ‘\n’ into “\r\n”

If you want to use only ‘\n’ instead of “\r\n” for printing a new line using printf(), define the macro CY_RETARGET_IO_CONVERT_LF_TO_CRLF using the DEFINES variable in the application Makefile. The library will then append ‘\r’ before ‘\n’ character on the output direction (STDOUT). No conversion occurs if “\r\n” is already present.

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