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Version 1.1: uart.vhd (13.1 KB)
Corrected rx start bit error checking

Version 1.0: uart_v1_0.vhd (13.0 KB)
Initial Public Release


  • VHDL source code of a Universal Asynchronous Receiver/Transmitter (UART) component
  • Full duplex
  • Configurable baud rate
  • Configurable data width
  • Configurable parity (even/odd/none)
  • Configurable oversampling rate for receive data
  • No flow control


This details a UART component for use in CPLDs and FPGAs, written in VHDL. The component was designed using Quartus II, version 13.1.0. Resource requirements depend on the implementation. Figure 1 illustrates a typical example of the UART integrated into a system.


Figure 1. Example Implementation


A UART is a device used for asynchronous serial communication. It consists of two lines for data transmission, RX and TX, one in each direction. Sometimes additional lines are included to implement flow control, most commonly RTS (Ready to Send) and CTS (Clear to Send). The transmission speed, data width, parity, and flow control are all configurable and must be set the same for both UART correspondents.

Figure 2 show the data framing for a transmission. Absent communication, the line is held high to indicate that it and the transmitter are not damaged. A transaction begins with a low start bit. The data word follows. Next comes an optional parity bit, which can be configured to even parity, odd parity, or no parity. Finally, a high stop bit ends the transaction.

Figure 2. Data Framing

The communication can be simplex, half duplex, or full duplex. In this component, the transmit and receive lines operate independently. While this makes the component inherently full duplex, it can also operate as either simplex or half duplex if controlled by the user logic to do so.

Theory of Operation

Generating Baud Rate and Oversampling Rate Clock Enables

The baud rate is the transmission speed of the data in bits/second. The oversampling rate is the number of times the receive circuitry samples the receive input per baud period (i.e. per data bit).

This component achieves the baud rate and oversampling rate by generating clock enable pulses at those frequencies. These signals are derived from the frequency of the system clock clk, which must be specified in the generic parameter clk_freq.

A counter within the component generates a baud pulse that occurs at the baud rate. This periodic pulse enables the system clock to operate the transmit circuitry at the baud rate. There is a small error introduced each baud period, since the generated baud rate must be an integer multiple of the system clock. However, this error will not exceed one period of the system clock.

Similarly, another counter generates an oversampling pulse that occurs at a frequency = oversampling rate * baud rate. This pulse enables the system clock to operate the receive circuitry at the oversampling rate. Note that this counter is also reset at each baud pulse, so that counting errors do not accumulate beyond one baud period. In this manner, the achieved baud rate of the receive circuitry is identical to the transmit circuitry baud rate.

Transmit Circuitry

When the tx_ena input is asserted, the data on tx_data is latched into an internal shift register. At this time, the parity bit is also calculated using XOR logic and latched into the same shift register, along with the start and stop bits. The baud pulse then periodically enables the system clock to shift out the register contents to the tx line at the baud rate. The tx_busy output indicates to the user logic when the transmission is complete and the circuitry is ready to accept new data to send.

Receive Circuitry

The receive circuitry monitors the rx input on each oversampling pulse. If it detects a logic low, it starts counting and recognizes an incoming start bit once it detects a sufficient number of consecutive low inputs. At this point, it begins shifting the value of the rx line into a shift register at the baud rate, achieved by counting oversampling pulses.

Once the entire data word is shifted into the register, the receive circuitry verifies the data parity using XOR logic. It then outputs the received data on the rx_data port and flags any error detected on the rx_error port. A high-to-low transition on the rx_busy port signifies to the user logic that new receive data is now available.

Configuring the UART

The UART is configured by setting the GENERIC parameters in the ENTITY. Table 1 describes the parameters.

Table 1. Generic Parameter Descriptions

Port Descriptions

Table 2 describes the UART’s ports.

Table 2. Port Descriptions


Although part of the same component, transmit and receive transactions don’t affect one another and therefore operate independently.


Figure 3 shows the timing diagram of an example transmission. Note that the baud_tb signal shown is internal to the UART and is included here only for purposes of explanation. The user does not need to consider this signal to use the UART.

Figure 3. Example Transmit Timing Diagram

A low logic level on the tx_busy output port indicates that the component is ready to send data. The component latches in the data presented on the tx_data port on the first rising edge of the clk where the tx_ena input is asserted. On the following clk, the component asserts the tx_busy signal to indicate that the UART transmit circuit is busy. Any inputs to the transmit circuitry during this condition are ignored. The actual transmission begins once the next baud period pulse baud_tb is generated within the UART. At this point, the tx signal outputs the logic low start bit and the transmission is underway. The UART transmits the least significant bit (LSB) of tx_data first. In the transaction shown, an even parity bit follows the data word. Once the logic high stop bit is sent and the transmission is complete, the tx_busy signal deasserts, indicating that the transmit circuitry is ready for the next transaction.


Figure 4 depicts the timing diagram of an example reception. A receive transaction is initiated by the UART’s correspondent via a logic low start bit at the rx input port. Once the UART detects that the rx input line has been low for half of a baud period, it recognizes the start bit and asserts the rx_busy signal to notify the user logic that a receive transaction is in progress. It proceeds to shift in the bits at the predetermined baud rate, calculate and verify the parity bit, and ensure that a logic high stop bit occurs at the correct time. Once it detects the stop bit, it outputs the received data to the rx_data port, with the first bit received as the LSB. It also outputs the rx_error signal to indicate whether or not a start bit, parity bit, or stop bit error was detected. It simultaneously deasserts the rx_busy signal to inform the user logic that the receive transaction is complete and that the results are available on the rx_data and rx_error ports.

Figure 4. Example Receive Timing Diagram

Figure 5 shows how the UART detects incoming bits on the receive line. The UART monitors the rx input at each oversampling pulse. In this simulation, the oversampling rate (os_rate) is set to 16, so 16 samples occur per baud period. Once the UART detects that the rx input is logic low for 8 consecutive samples (os_rate/2), it determines that a start bit is present. It then shifts in a data bit every 16th sample, which corresponds to the middle of each receive bit. The higher the oversampling rate, the closer to the middle of the bit this is guaranteed to be.

Figure 5. Receive Bit Reception Within the UART


The reset_n input port must have a logic high for the UART component to operate. A low signal on this port asynchronously resets the component. During reset, the component holds the tx_busy port high to indicate that it is not available to send data. Any transmit currently underway is discontinued, and the tx output assumes a logic high state. The rx_busy port is held low to indicate that no receive is in progress. Any receive currently in progress is abandoned, and the rx_data and rx_error output ports clear. Once released from reset, the tx_busy port deasserts on the following clk, indicating the UART’s readiness to communicate.


This UART is a configurable programmable logic component that accommodates communication through a simple asynchronous serial interface. It allows a user to specify the system clock, baud rate, data length, parity scheme, and oversampling rate.


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