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Retarget IO

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 when configuring the UART.

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.

NOTE: In general, console prints such as printf() should not be performed in ISR context. It must definitely not be called in ISR context when CY_RTOS_AWARE is defined, as the threat safety implementation disallows such calls.

NOTE: If the application is built without HAL support (i.e., neither COMPONENT_MTB_HAL nor CY_USING_HAL is defined), then the UART must be initialized using PDL function calls prior to being passed into the cy_retarget_io_init() function. See Quick Start (PDL Only) section below.

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.

HAL support is required for retarget-io in an RTOS environment. If your project does not include HAL support, you must manually manage concurrency in your application.

Quick Start (with MTB-HAL(COMPONENT_MTB_HAL) Support)

  1. Configure the UART using the device configurator generated structures or through manually written config structures. Configuration includes the UART TX and RX pins, CTS/RTS pins if flow control is desired, Baud Rate and other UART config parameters
  2. Set up the clock source to the UART peripheral. This could be done using the device configurator or manually. Set up the clock divider value depending on the desired baud rate.
  3. Initialize the UART Hardware.
  4. Set up the HAL UART object.
  5. Add #include "cy_retarget_io.h".
  6. Call cy_retarget_io_init(&hal_uart_obj);.
  7. Start printing using printf().

Quick Start (with CY-HAL(CY_USING_HAL) Support)

  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()

Quick Start (PDL Only)

These instructions apply when the UART interface is provided by the SCB peripheral directly using a PDL configured and initialized SCB object. Only relevant when HAL is unavailable. UART interfaces other than SCB are not supported at this time.

  1. Add #include "cy_retarget_io.h"
  2. Initialize and enable your UART hardware using PDL function calls. The DEBUG_UART must be defined and configured in the Device Configurator tool.
    Cy_SCB_UART_Init(DEBUG_UART_HW, &DEBUG_UART_config, NULL);
    Cy_SCB_UART_Enable(DEBUG_UART_HW);
  1. Call cy_retarget_io_init(DEBUG_UART_HW);
  2. 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.

Floating Point Support

By default, floating point support is enabled in printf. If floating point values will not be used in printed strings, this functionality can be disabled to reduece flash consumption. To disable floating support, add the following to the application makefile: DEFINES += CY_RETARGET_IO_NO_FLOAT.

More information

© Cypress Semiconductor Corporation (an Infineon company) or an affiliate of Cypress Semiconductor Corporation, 2019-2024.