Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 7/19/2024
Public

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5.13.6.2. UART Serial Protocol

Because the serial communication between the UART controller and the selected device is asynchronous, additional bits (start and stop) are added to the serial data to indicate the beginning and end. Utilizing these bits allows two devices to be synchronized. This structure of serial data accompanied by start and stop bits is referred to as a character, as shown below.

An additional parity bit may be added to the serial character. This bit appears after the last data bit and before the stop bit(s) in the character structure to provide the HPS UART controllers controller with the ability to perform simple error checking on the received data.

The Control Register is used to control the serial character characteristics. The individual bits of the data word are sent after the start bit, starting with the least significant bit (LSB). These are followed by the optional parity bit, followed by the stop bit(s), which can be 1, 1.5 or 2.

All the bits in the transmission (with exception to the half stop bit when 1.5 stop bits are used) are transmitted for exactly the same time duration. This is referred to as a Bit Period or Bit Time. One Bit Time equals 16 baud clocks. To ensure stability on the line, the receiver samples the serial input data at approximately the midpoint of the Bit Time once the start bit has been detected. Because the exact number of baud clocks that each bit transmission is known, calculating the midpoint for sampling is not difficult. That is, every 16 baud clocks after the midpoint sample of the start bit.

Together with serial input debouncing, this feature also contributes to avoid the detection of false start bits. Short glitches are filtered out by debouncing, and no transition is detected on the line. If a glitch is wide enough to avoid filtering by debouncing, a falling edge is detected. However, a start bit is detected only if the line is sampled low again after half a bit time has elapsed.

Figure 245. Receiver Serial Data Sample Points

The baud rate of the UART controller is controlled by the serial clock and the Divisor Latch Register ( DLH and DLL ).