group group_scb_uart

Driver API for UART.

The functions and other declarations used in this part of the driver are in cy_scb_uart.h. You can also include cy_pdl.h to get access to all functions and declarations in the PDL.

The Universal Asynchronous Receiver/Transmitter (UART) protocol is an asynchronous serial interface protocol. UART communication is typically point-to-point. The UART interface consists of two signals:

  • TX: Transmitter output

  • RX: Receiver input

Additionally, two side-band signals are used to implement flow control in UART. Note that the flow control applies only to TX functionality.

  • Clear to Send (CTS): This is an input signal to the transmitter. When active, it indicates that the slave is ready for the master to transmit data.

  • Ready to Send (RTS): This is an output signal from the receiver. When active, it indicates that the receiver is ready to receive data


  • Supports UART protocol

    • Standard UART

    • Multi-processor mode

  • SmartCard (ISO7816) reader

  • IrDA

  • Data frame size programmable from 4 to 16 bits

  • Programmable number of STOP bits, which can be set in terms of half bit periods between 1 and 4

  • Parity support (odd and even parity)

  • Median filter on Rx input

  • Programmable oversampling

  • Start skipping

Configuration Considerations

The UART driver configuration can be divided to number of sequential steps listed below:

  • Configure UART

  • Assign and Configure Pins

  • Assign Clock Divider

  • Configure Baud Rate

  • Configure Interrupt

  • Enable UART


UART driver is built on top of the SCB hardware block. The SCB5 instance is used as an example for all code snippets. Modify the code to match your design.

Configure UART

To set up the UART driver, provide the configuration parameters in the cy_stc_scb_uart_config_t structure. For example: provide uartMode, oversample, dataWidth, enableMsbFirst, parity, and stopBits. The other parameters are optional. To initialize the driver, call Cy_SCB_UART_Init function providing a pointer to the populated cy_stc_scb_uart_config_t structure and the allocated cy_stc_scb_uart_context_t structure.

/* Allocate context for UART operation */
cy_stc_scb_uart_context_t uartContext;

/* Populate configuration structure */
const cy_stc_scb_uart_config_t uartConfig =
    .uartMode                   = CY_SCB_UART_STANDARD,
    .enableMutliProcessorMode   = false,
    .smartCardRetryOnNack       = false,
    .irdaInvertRx               = false,
    .irdaEnableLowPowerReceiver = false,

    .oversample                 = 12UL,

    .enableMsbFirst             = false,
    .dataWidth                  = 8UL,
    .parity                     = CY_SCB_UART_PARITY_NONE,
    .stopBits                   = CY_SCB_UART_STOP_BITS_1,
    .enableInputFilter          = false,
    .breakWidth                 = 11UL,
    .dropOnFrameError           = false,
    .dropOnParityError          = false,

    .receiverAddress            = 0UL,
    .receiverAddressMask        = 0UL,
    .acceptAddrInFifo           = false,

    .enableCts                  = false,
    .ctsPolarity                = CY_SCB_UART_ACTIVE_LOW,
    .rtsRxFifoLevel             = 0UL,
    .rtsPolarity                = CY_SCB_UART_ACTIVE_LOW,

    .rxFifoTriggerLevel  = 0UL,
    .rxFifoIntEnableMask = 0UL,
    .txFifoTriggerLevel  = 0UL,
    .txFifoIntEnableMask = 0UL,

/* Configure UART to operate */
(void) Cy_SCB_UART_Init(SCB5, &uartConfig, &uartContext);

Assign and Configure Pins

Only dedicated SCB pins can be used for UART operation. The HSIOM register must be configured to connect dedicated SCB UART pins to the SCB block. Also, the UART output pins must be configured in Strong Drive Input Off mode and UART input pins in Digital High-Z:

/* Assign pins for UART on SCB5: P5[0], P5[1] */
#define UART_PORT       P5_0_PORT
#define UART_RX_NUM     P5_0_NUM
#define UART_TX_NUM     P5_1_NUM

/* Connect SCB5 UART function to pins */

/* Configure pins for UART operation */

Assign Clock Divider

A clock source must be connected to the SCB block to oversample input and output signals, in this document this clock will be referred as clk_scb. You must use one of available integer or fractional dividers. Use the SysClk (System Clock) driver API to do this.

/* Assign divider type and number for UART */
#define UART_CLK_DIV_NUMBER   (0U)

/* Connect assigned divider to be a clock source for UART */

Configure Baud Rate

To get the UART to operate with the desired baud rate, the clk_scb frequency and the oversample must be configured. Use the SysClk (System Clock) driver API to configure clk_scb frequency. Set the to define the number of the SCB clocks within one UART bit-time.

/* UART desired baud rate is 115200 bps (Standard mode).
* The UART baud rate = (clk_scb / Oversample).
* For clk_peri = 50 MHz, select divider value 36 and get SCB clock = (50 MHz / 36) = 1,389 MHz.
* Select Oversample = 12. These setting results UART data rate = 1,389 MHz / 12 = 115750 bps.
Cy_SysClk_PeriphSetDivider   (UART_CLK_DIV_TYPE, UART_CLK_DIV_NUMBER, 35UL);
Refer to the technical reference manual (TRM) section UART sub-section Clocking and Oversampling to get information about how to configure the UART to run with desired baud rate.

Configure Interrupt

The interrupt is optional for the UART operation. To configure interrupt the Cy_SCB_UART_Interrupt function must be called in the interrupt handler for the selected SCB instance. Also, this interrupt must be enabled in the NVIC. The interrupt must be configured when High-Level API will be used.

void UART_Isr(void)
    Cy_SCB_UART_Interrupt(SCB5, &uartContext);
/* Assign UART interrupt number and priority */
#define UART_INTR_NUM        ((IRQn_Type) scb_5_interrupt_IRQn)

/* Populate configuration structure (code specific for CM4) */
cy_stc_sysint_t uartIntrConfig =
    .intrSrc      = UART_INTR_NUM,
    .intrPriority = UART_INTR_PRIORITY,

/* Hook interrupt service routine and enable interrupt */
(void) Cy_SysInt_Init(&uartIntrConfig, &UART_Isr);

Enable UART

Finally, enable the UART operation by calling Cy_SCB_UART_Enable.

/* Enable UART to operate */

/* Enable global interrupts */

Common Use Cases

The UART API is divided into two categories: Low-Level and High-Level. Do not mix and API because a Low-Level API can adversely affect the operation of a High-Level API.

Low-Level API

The Low-Level functions allow interacting directly with the hardware and do not use Cy_SCB_UART_Interrupt. These functions do not require context for operation. Thus, NULL can be passed for context parameter in Cy_SCB_UART_Init and Cy_SCB_UART_Disable instead of a pointer to the context structure.

uint8_t txBuffer[BUFFER_SIZE];

/* Initialize txBuffer with command to transfer */
txBuffer[1] = 0x00U;
txBuffer[2] = 0x01U;

/* Master: start a transfer. Slave: prepare for a transfer. */
Cy_SCB_UART_PutArrayBlocking(SCB5, txBuffer, sizeof(txBuffer));

/* Blocking wait for transfer completion */
while (!Cy_SCB_UART_IsTxComplete(SCB5))

High-Level API

The High-Level API use Cy_SCB_UART_Interrupt to execute the transfer. Call Cy_SCB_UART_Transmit to start transmission. Call Cy_SCB_UART_Receive to start receive operation. After the operation is started the Cy_SCB_UART_Interrupt handles the data transfer until its completion. Therefore Cy_SCB_UART_Interrupt must be called inside the user interrupt handler to make the High-Level API work. To monitor status of transmit operation, use Cy_SCB_UART_GetTransmitStatus and Cy_SCB_UART_GetReceiveStatus to monitor receive status appropriately. Alternatively use Cy_SCB_UART_RegisterCallback to register callback function to be notified about UART Callback Events.

Receive Operation

uint8_t rxBuffer[BUFFER_SIZE];

/* Start receive operation (do not check status) */
(void) Cy_SCB_UART_Receive(SCB5, rxBuffer, sizeof(rxBuffer), &uartContext);

/* Blocking wait until buffer is full */
while (0UL != (CY_SCB_UART_RECEIVE_ACTIVE & Cy_SCB_UART_GetReceiveStatus(SCB5, &uartContext)))

/* Handle received data */
Transmit Operation
uint8_t txBuffer[BUFFER_SIZE];

/* Initialize txBuffer with data to transfer */
txBuffer[1] = 0x00U;
txBuffer[2] = 0x01U;

/* Start transmit operation (do not check status) */
(void) Cy_SCB_UART_Transmit(SCB5, txBuffer, sizeof(txBuffer), &uartContext);

/* Blocking wait for transmission completion */
while (0UL != (CY_SCB_UART_TRANSMIT_ACTIVE & Cy_SCB_UART_GetTransmitStatus(SCB5, &uartContext)))
There is also capability to insert a receive ring buffer that operates between the RX FIFO and the user buffer. The received data is copied into the ring buffer from the RX FIFO. This process runs in the background after the ring buffer operation is started by Cy_SCB_UART_StartRingBuffer. When Cy_SCB_UART_Receive is called, it first reads data from the ring buffer and then sets up an interrupt to receive more data if the required amount has not yet been read.

DMA Trigger

The SCB provides TX and RX output trigger signals that can be routed to the DMA controller inputs. These signals are assigned based on the data availability in the TX and RX FIFOs appropriately.

  • The RX trigger signal is active while the number of data elements in the RX FIFO is greater than the value of RX FIFO level. Use function Cy_SCB_SetRxFifoLevel or set configuration structure rxFifoTriggerLevel parameter to configure RX FIFO level value. For example, the RX FIFO has 8 data elements and the RX FIFO level is 0. The RX trigger signal is active until DMA reads all data from the RX FIFO.

  • The TX trigger signal is active while the number of data elements in the TX FIFO is less than the value of TX FIFO level. Use function Cy_SCB_SetTxFifoLevel or set configuration structure txFifoTriggerLevel parameter to configure TX FIFO level value. For example, the TX FIFO has 0 data elements (empty) and the TX FIFO level is 7. The TX trigger signal is active until DMA loads TX FIFO with 8 data elements (note that after the first TX load operation, the data element goes to the shift register and TX FIFO is empty).

To route SCB TX or RX trigger signals to DMA controller use TrigMux (Trigger Multiplexer) driver API.


To properly handle DMA level request signal activation and de-activation from the SCB peripheral block the DMA Descriptor typically must be configured to re-trigger after 16 Clk_Slow cycles.

Low Power Support

The UART driver provides callback functions to handle power mode transition. The callback Cy_SCB_UART_DeepSleepCallback must be called during execution of Cy_SysPm_CpuEnterDeepSleep Cy_SCB_UART_HibernateCallback must be called during execution of Cy_SysPm_SystemEnterHibernate. To trigger the callback execution, the callback must be registered before calling the power mode transition function. Refer to SysPm (System Power Management) driver for more information about power mode transitions and callback registration.

The UART is disabled during Deep Sleep and Hibernate and stops driving the output pins. The state of the UART output pins TX and RTS is High-Z, which can cause unexpected behavior of the UART receiver due to possible glitches on these lines. These pins must be set to the inactive state before entering Deep Sleep or Hibernate mode. These pins must keep the inactive level (the same state when UART TX is enabled and does not transfer data) before entering Deep Sleep or Hibernate mode. To do that, write the GPIO data register of each pin to the inactive level for each output pin. Then configure High-Speed Input Output Multiplexer (HSIOM) of each pin to be controlled by the GPIO (use GPIO (General Purpose Input Output) driver API). After exiting Deep Sleep mode the UART must be enabled and the pins configuration restored to return the UART control of the pins (after exiting Hibernate mode, the system initialization code does the same). Copy either or both Cy_SCB_UART_DeepSleepCallback and Cy_SCB_UART_HibernateCallback as appropriate, and make the changes described above inside the function. Alternately, external pull-up or pull-down resistors can be connected to the appropriate UART lines to keep them inactive during Deep-Sleep or Hibernate.