FreeRTOS_read()

[FreeRTOS-Plus-IO API]

FreeRTOS_IO.h

size_t FreeRTOS_read( Peripheral_Descriptor_t const pxPeripheral, 
                      void * const pvBuffer, 
                      const size_t xBytes );

从打开的外围设备读取一个或多个字节。

板级支持包定义了可以打开的外围设备。 FreeRTOS_ioctl() 用于在中断驱动读取模式和轮询读取模式之间进行选择。

参数：

pxPeripheral	与外围设备相关联的描述符，从该设备中读取字节。描述符将从用于开启外围设备的 FreeRTOS_open() 调用中返回。
pvBuffer	存放读取数据的缓冲区。
xBytes	请求的总字节数。当使用中断驱动传输模式时，如果请求的总数在外围设备读取超时值逾时前无法获取，则实际读取的字节总数将小于请求的总数。 FreeRTOS_ioctl() 用于设置读取超时值。

返回：

读取的字节总数。如果请求的字节数在外围设备读取超时值逾时前无法读取，那么读取的字节总数将小于由 xBytes 参数请求的字节数。 FreeRTOS_ioctl() 用于设置读取超时值。

用法示例：

示例 1 代码片段演示了当外围设备被配置为使用轮询传输模式时，应如何进行读取操作。打开外围设备时默认采用轮询模式。



/* By default the port is opened in polled mode.  Read sizeof( ucBuffer ) bytes into

ucBuffer using polled mode. */

xBytesRead = FreeRTOS_read( xPort, ucBuffer, sizeof( ucBuffer ) );



/* The port is currently in polled mode, so FreeRTOS_read() will only have

returned once all the requested bytes had been read (barring any errors on

the peripheral).  Note that, because polling mode is being used, the task

making the FreeRTOS_read() call will not have entered the Blocked

state if it had to wait for the requested number of bytes. */

configASSERT( xBytes == sizeof( ucBuffer ) );



Example 1: Reading bytes from a peripheral that is configured to use the polled transfer mode.

示例 2 代码片段演示了当外围设备被配置为使用中断驱动字符队列或传输模式，或中断驱动的循环缓冲区传输模式时，应如何进行读取操作。在这些模式下，执行 FreeRTOS_read() 调用的任务维持在阻塞状态（不使用任何 CPU 时间），直到已读取请求字节数量，或读取超时值已逾期。 FreeRTOS_ioctl() 使用 iocltSET_RX_TIMEOUT 请求代码来配置读取超时值。

    

/* Read some bytes in one of the interrupt driven transfer modes.  If the 

character queue transfer mode is being used, this will remove bytes from the 

queue that had previously been placed into the queue by the FreeRTOS-Plus-IO interrupt 

service routine.  If the circular buffer transfer mode is being used, this will 

remove bytes from the circular buffer that had previously been placed into the 

buffer by the FreeRTOS-Plus-IO interrupt service routine.  In both cases, read bytes 

are placed in ucBuffer. */

xBytesRead = FreeRTOS_read( xPort, ucBuffer, sizeof( ucBuffer ) );



if( xBytesRead < sizeof( ucBuffer ) )

{

    /* The Rx timeout must have expired before sizeof( ucBuffer ) bytes could

    be read. xBytesRead number of bytes will have been placed into ucBuffer. */

}

else

{

    /* The requested number of bytes were read before the read timeout expired.  

    All the requested bytes have been placed in ucBuffer. */

}



Example 2: Reading bytes from a peripheral that is configured to used either the


interrupt driven character queue transfer mode, or the interrupt driven circular


buffer transfer mode.

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FreeRTOS_read() [FreeRTOS-Plus-IO API]

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FreeRTOS_read()

[FreeRTOS-Plus-IO API]