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OSR 最小值是 4，因此24Mhz時鐘模式下，LPUART最大波特率是 24/5 = 4.8Mbps，手冊中硬件的 LPUART最大的波特率是 80Mhz/4=20Mbps。80/13= 6.15Mbps波特率，這樣相對于 6Mhz就有 2.5%的誤差。

那如果用 FlexIO外設實現 UART波特率可以達到 6Mbps么？

將FlexIO用于UART RX時，RM參考手冊的注意事項：“FlexIO數據只在每個位的中間采樣一次。可以使用另一個定時器對傳入數據進行毛刺過濾”。保持FlexIO時鐘與RM參考手冊中的波特率的倍數相同。RM參考手冊中的UART接收和UART發射配置表將所使用的每個FlexIO定時器的TIMCMP設置為0xF01。這意味著波特率是FlexIO時鐘頻率的1/4。FlexIO不支持奇偶校驗位的自動驗證。

使用了SDK中的Flexio_uart示例，并使用了flexio_uart驅動程序，只對這些頻率/波特率進行了一些細微的更改。示例使用480MHz的PLL3作為FlexIO的時鐘源。將FLEXIO2_CLK_PRED除法器設置為（4+1），將FLEXIO2_CLK_PODF除法器設為（3+1），從而提供 24MHz（480/5/4）的FlexIO時鐘。對于驅動程序，將flexio_uart_config_t.baudRate_Bps設置為6000000。然后，驅動程序計算兩個定時器的TIMCMP寄存器=0xF01。

它在IMXRT1060-EVKB板上運行，SDK v2.12.1，如下代碼簡單修改。SDK_2_12_1_MIMXRT1062xxxxBoardsevkbmimxrt1060driver_examplesflexiouartedma_transfer 此應用程序僅在引腳GPIO_B0_05上傳輸，RX設置在引腳GPIO_B0_06上。

IOMUXC_SetPinMux(IOMUXC_GPIO_B0_05_FLEXIO2_FLEXIO05, 0U);

IOMUXC_SetPinMux(IOMUXC_GPIO_B0_06_FLEXIO2_FLEXIO06, 0U);

IOMUXC_SetPinConfig(IOMUXC_GPIO_B0_05_FLEXIO2_FLEXIO05, 0x10B0U);

IOMUXC_SetPinConfig(IOMUXC_GPIO_B0_06_FLEXIO2_FLEXIO06, 0x10B0U);

#include "pin_mux.h"

#include "clock_config.h"

#include "board.h"

#include "fsl_flexio_uart_edma.h"

#include "fsl_dmamux.h"

#define BOARD_FLEXIO_BASE FLEXIO2

#define FLEXIO_UART_TX_PIN 5U

#define FLEXIO_UART_RX_PIN 6U

/* Select USB1 PLL (480 MHz) as flexio clock source */

#define FLEXIO_CLOCK_SELECT (3U)

/* Clock pre divider for flexio clock source */

#define FLEXIO_CLOCK_PRE_DIVIDER (4U)

/* Clock divider for flexio clock source */

#define FLEXIO_CLOCK_DIVIDER (3U)

#define FLEXIO_CLOCK_FREQUENCY

(CLOCK_GetFreq(kCLOCK_Usb1PllClk) / (FLEXIO_CLOCK_PRE_DIVIDER + 1U) / (FLEXIO_CLOCK_DIVIDER + 1U))

#define FLEXIO_DMA_REQUEST_BASE kDmaRequestMuxFlexIO2Request0Request1

#define EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR DMAMUX

#define EXAMPLE_FLEXIO_UART_DMA_BASEADDR DMA0

#define FLEXIO_UART_TX_DMA_CHANNEL 0U

#define FLEXIO_UART_RX_DMA_CHANNEL 1U

#define FLEXIO_TX_SHIFTER_INDEX 0U

#define FLEXIO_RX_SHIFTER_INDEX 2U

#define EXAMPLE_TX_DMA_SOURCE kDmaRequestMuxFlexIO2Request0Request1

#define EXAMPLE_RX_DMA_SOURCE kDmaRequestMuxFlexIO2Request2Request3

#define ECHO_BUFFER_LENGTH 8

void FLEXIO_UART_UserCallback(FLEXIO_UART_Type *base,

flexio_uart_edma_handle_t *handle,

status_t status,

void *userData);

flexio_uart_edma_handle_t g_uartHandle;

FLEXIO_UART_Type uartDev;

edma_handle_t g_uartTxEdmaHandle;

edma_handle_t g_uartRxEdmaHandle;

AT_NONCACHEABLE_SECTION_INIT(uint8_t g_tipString[]) =

"Flexio uart edma example Board receives 8 characters then sends them out Now please input: ";

AT_NONCACHEABLE_SECTION_INIT(uint8_t g_txBuffer[ECHO_BUFFER_LENGTH]) = {0};

AT_NONCACHEABLE_SECTION_INIT(uint8_t g_rxBuffer[ECHO_BUFFER_LENGTH]) = {0};

volatile bool rxBufferEmpty = true;

volatile bool txBufferFull = false;

volatile bool txOnGoing = false;

volatile bool rxOnGoing = false;

/* UART 用戶回調函數*/

void FLEXIO_UART_UserCallback(FLEXIO_UART_Type *base,

flexio_uart_edma_handle_t *handle,

status_t status,

void *userData)

{

userData = userData;

if (kStatus_FLEXIO_UART_TxIdle == status)

{

txBufferFull = false;

txOnGoing = false;

}

if (kStatus_FLEXIO_UART_RxIdle == status)

{

rxBufferEmpty = false;

rxOnGoing = false;

}

}

int main(void)

{

flexio_uart_config_t userconfig;

flexio_uart_transfer_t xfer;

flexio_uart_transfer_t sendXfer;

status_t result = kStatus_Success;

edma_config_t config;

BOARD_ConfigMPU();

BOARD_InitPins();

BOARD_BootClockRUN();

/* Flexio 時鐘設置*/

CLOCK_SetMux(kCLOCK_Flexio2Mux, FLEXIO_CLOCK_SELECT);

CLOCK_SetDiv(kCLOCK_Flexio2PreDiv, FLEXIO_CLOCK_PRE_DIVIDER);

CLOCK_SetDiv(kCLOCK_Flexio2Div, FLEXIO_CLOCK_DIVIDER);

/*

* config.enableUart = true;

* config.enableInDoze = false;

* config.enableInDebug = true;

* config.enableFastAccess = false;

* config.bitCountPerChar = kFLEXIO_UART_8BitsPerChar;

*/

FLEXIO_UART_GetDefaultConfig(&userconfig);

userconfig.baudRate_Bps = BOARD_DEBUG_UART_BAUDRATE;

userconfig.enableUart = true;

uartDev.flexioBase = BOARD_FLEXIO_BASE;

uartDev.TxPinIndex = FLEXIO_UART_TX_PIN;

uartDev.RxPinIndex = FLEXIO_UART_RX_PIN;

uartDev.shifterIndex[0] = FLEXIO_TX_SHIFTER_INDEX;

uartDev.shifterIndex[1] = FLEXIO_RX_SHIFTER_INDEX;

uartDev.timerIndex[0] = 0U;

uartDev.timerIndex[1] = 1U;

result = FLEXIO_UART_Init(&uartDev, &userconfig, FLEXIO_CLOCK_FREQUENCY);

if (result != kStatus_Success)

{

return -1;

}

/*初始化 DMA*/

DMAMUX_Init(EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR);

EDMA_GetDefaultConfig(&config);

EDMA_Init(EXAMPLE_FLEXIO_UART_DMA_BASEADDR, &config);

/* 為 TX&RX 設置 DMA 通道*/

DMAMUX_SetSource(EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR, FLEXIO_UART_TX_DMA_CHANNEL, EXAMPLE_TX_DMA_SOURCE);

DMAMUX_SetSource(EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR, FLEXIO_UART_RX_DMA_CHANNEL, EXAMPLE_RX_DMA_SOURCE);

DMAMUX_EnableChannel(EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR, FLEXIO_UART_TX_DMA_CHANNEL);

DMAMUX_EnableChannel(EXAMPLE_FLEXIO_UART_DMAMUX_BASEADDR, FLEXIO_UART_RX_DMA_CHANNEL);

EDMA_CreateHandle(&g_uartTxEdmaHandle, EXAMPLE_FLEXIO_UART_DMA_BASEADDR, FLEXIO_UART_TX_DMA_CHANNEL);

EDMA_CreateHandle(&g_uartRxEdmaHandle, EXAMPLE_FLEXIO_UART_DMA_BASEADDR, FLEXIO_UART_RX_DMA_CHANNEL);

FLEXIO_UART_TransferCreateHandleEDMA(&uartDev, &g_uartHandle, FLEXIO_UART_UserCallback, NULL, &g_uartTxEdmaHandle,&g_uartRxEdmaHandle);

/* 發送g_tipString */

xfer.data = g_tipString;

xfer.dataSize = sizeof(g_tipString) - 1;

txOnGoing = true;

FLEXIO_UART_TransferSendEDMA(&uartDev, &g_uartHandle, &xfer);

/* 等待發送完成 */

while (txOnGoing)

{

}

/* 開始應答 */

sendXfer.data = g_txBuffer;

sendXfer.dataSize = ECHO_BUFFER_LENGTH;

while (1)

{

/* If TX is idle and g_txBuffer is full, start to send data. */

if ((!txOnGoing) && txBufferFull)

{

txOnGoing = true;

FLEXIO_UART_TransferSendEDMA(&uartDev, &g_uartHandle, &sendXfer);

}

/* 如果 g_txBuffer為空，加載 g_txBuffer */

if (!txBufferFull)

{

memcpy(g_txBuffer, "UUUUUUUU", ECHO_BUFFER_LENGTH);

txBufferFull = true;

}

}

}