UART interrupt handling is a common technique for microcontroller-based applications that need to communicate with other devices via serial ports. In this article, you will learn what UART interrupt-driven communication is, how it works, and what are its advantages and disadvantages compared to polling or DMA methods.
UART stands for Universal Asynchronous Receiver-Transmitter, and it is a hardware module that allows serial data transmission and reception between two devices. Interrupt-driven communication means that the microcontroller can perform other tasks while waiting for a UART event, such as a byte being received or transmitted. When such an event occurs, the UART module generates an interrupt request, which signals the microcontroller to pause its current task and execute a specific interrupt service routine (ISR) that handles the UART operation.
There are few more advantages of Interrupt based on UART compare to polling based mechanism - Multitask communication can be done based on Interrupt as it can be executed easily compared to polling based mechanism - Interrupts can be served easily which consumes less power compared polling based mechanism
Advantages of UART Interrupt-Driven Communication, - Efficiency: Interrupts allow asynchronous communication, enabling the processor to perform other tasks while waiting for data. - Low CPU Utilization: The CPU is freed from continuous polling, conserving resources for other operations. Disadvantages, - Overhead: Handling interrupts introduces overhead, impacting overall system performance. - Complexity: Implementing interrupt-driven communication requires careful management, making the code more complex than simple polling or DMA approaches.
Overall polling is used for quick and dirty stuff. A real and robust architecture uses interrupt driven, for almost every i/o, not only for communication buses.
To use UART interrupt-driven communication, you need to configure the UART module and enable the corresponding interrupt sources. For example, you can enable the receive interrupt, which will trigger an ISR whenever a byte is received in the UART buffer. Similarly, you can enable the transmit interrupt, which will trigger an ISR whenever the UART buffer is empty and ready to send a new byte. In the ISR, you can read or write data to the UART buffer, and perform any other actions related to the UART operation. After the ISR is completed, the microcontroller returns to its previous task.
UART module generates interrupts to handle incoming and outgoing data, allowing the processor to perform other tasks while data is being transmitted or received. Following steps needed to be carefully carried out to achieve this, 1. Interrupt Enablement and Configuration 2. Buffer Management 3. Interrupt Service Routine (ISR) 4. Receiving Data / Transmitting Data 5. Flow Control (Optional) 6. Non-Blocking Operation In summary, it provides a flexible and efficient way to handle serial data transfer. By leveraging interrupts, microcontroller can perform other tasks while UART module takes care of data transmission and reception. Proper management of buffers and flow control mechanisms ensures smooth and reliable communication between devices.
One of the main advantages of UART interrupt-driven communication is that it allows the microcontroller to perform other tasks while communicating via serial ports, without wasting CPU cycles in polling the UART status or transferring data manually. This can improve the efficiency and responsiveness of your application, as well as reduce power consumption. Another advantage is that you can handle multiple UART events with different priorities, by using interrupt nesting or preemption features of the microcontroller. This can enable more complex and flexible serial communication scenarios.
All I/O really should be interrupt driven, even if you need a custom I/O interface such as fashing LEDs, keypad scanners, LCD interfaces etc. Set up timers, use state machines if necessary and keep ISRs short and quick. No blocking code should be in the main loop, and certainly not in ISRs. This allows changing the timing of an interface, or adding new I/O interfaces, without affecting the timing of other interfaces.
Using UART to handle ASCII data in particular results in easy-to-read code. On a microcontroller. the main data handler will result in a series of string comparisons against commands designed in the communication protocol. This allows for easy code maintenance and upgrades when new commands need to be added.
I'm not sure if I agree with "All" IO should be interrupt driven. There are devices that do NOT need immediate response and there're events that needs to be responded immediately (ie. power outage). The type of OS is important too (in terms of how it deals with task handling/switching). RTOS (real time OS) deals much better with interrupts than non RTOS (ie. Windows). I agree that no blocking code should be implemented everywhere except for critical tasks. I've seen applications that totally ignore ^C once it starts running (and it's not even critical task). How one decides whether to use interupt driven or polling depends on the application (UART included)... but in general, interrupt driven has better response time.
There's some conflicting information in this paragraph. The responsiveness will suffer, because there is ALWAYS a push, and a pop latency when handling an interrupt of any kind. Just as well, topics such as interrupt nesting and pre-emption make the assumption that some layer of pseudo-parallelism (eg RTOS) is running on the microcontroller, which is not always the case. One important thing to note is that almost all serial communication contains some sort of checksum validation. Thus, an MCU is very unlikely to carry out an actionable event without validating the data integrity of the message first. So the message must be received and interpretted in its entirety in order to validate and then execute.
One of the main disadvantages of UART interrupt-driven communication is that it introduces more complexity and overhead to your application, as you need to write and manage the ISR code, as well as handle possible errors and conflicts. For example, you need to ensure that the ISR code is fast and reliable, and that it does not interfere with other interrupts or tasks. Another disadvantage is that you need to allocate enough memory and buffer space for the UART data, and synchronize them with the ISR and the main program. This can increase the memory footprint and the risk of data loss or corruption.
I agree with the DMA comments, I've implemented a circular ring buffer to receive 115k baud RS422 packets in an Atmel XMEGA by configuring the DMA engine with practically no software overheads from interrupts. Similarly for outputing SPI data (in my case refreshing a multiplexed a 64x32 dot matrix LED display at 100Hz frame rate). DMA engines can significantly reduce the number and rate of interrupts the MCU has to deal with.
DMA utilization is the way to go. When done properly along with static allocation at an aligned block of memory (learn how the linker script works, along with gcc attributes), one can leverage DMA indexing to effectively create a dual-port FIFO. Thus the CPU can periodically check the FIFO for data, interpret the received data quickly, and validate and execute on cached data, rather than waiting around for something to happen, or pushing and popping its way to ineffectuality.
In my experience these disadvantages are not really impactful. The only time you don't want to use asynchronous UART communication is the only time you don't want to use _any_ asynchronous communication: when timing requirements are strict enough to require polling from a calibrated clock. I've never seen a UART application violate memory constraints; buffer size can be controlled through proper communication protocol design. A lightly written ISR (read byte here, push byte there) should execute within a few clock cycles.
UART interrupt-driven communication is suitable for applications that require frequent and asynchronous serial communication with other devices, and that can benefit from multitasking and low-power operation. For example, you can use UART interrupt-driven communication for sensor networks, wireless modules, GPS receivers, LCD displays, or user interfaces. However, if your application only needs simple and occasional serial communication, or if you have strict timing or performance requirements, you may consider using polling or DMA methods instead.
This is a very simplistic view of UART, interrupts and DMA. The reality is more complex. For example if you are sending data out a UART from interrupt handlers, then things get more complicated. For example imagine that you are doing syslog via UART and interrupt handler you log an error. Now if you are using UART driver via interrupts you have to make sure your UART interrupt is higher priority than all others else system could hang. DMA has problems in that if you use a UART putchar(char p) type of function then when do you kick off DMA? Do you kick off a DMA for each byte pushed to UART? Here you need to know when to flush the TX buffer to the UART via DMA.
I always use ISRs with my UARTs, absolutely without fail. All traffic goes through queues, which basically turn my hard real time constraints into soft real time constraints. My tasks then hang off the queues, a nice abstraction. Even when I have DMA available I don't use it because DMA transfers rarely have enough flexibility in terms of detecting the end of a message or of implementing circular buffers with overrun detection. You can also get an easy power saving win if you can idle the CPU until an interrupt is recognized.
With the M0+ becoming the very base model of modern MCUs, all equipped with capable DMA controllers, there is never a good reason to use ISRs for serial data other than ignorance or expedience on the part of the developer. Use ISRs if you need to do something quick and dirty. Or you need a wake-from-sleep condition for a companion chip. Otherwise, it's an outdated approach in 2023.
To use UART interrupt-driven communication in code, you need to follow some general steps. Initialize and configure the UART module by setting the baud rate, parity, stop bits, and data bits. Then enable the global and specific interrupts for the UART module. Define and implement the ISR functions for the UART interrupts, such as reading or writing data to the UART buffer, and handling errors or events. Finally, use the main program loop to perform other tasks while waiting for the UART interrupts. Here is a simple example of using UART interrupt-driven communication in C code for a microcontroller: first, initialize and enable the UART module and interrupts; second, perform other tasks; and third, toggle an LED every second.
C code example is referenced but actually missing. Seems like some text is repeated. Please remove this 125 characters limit also...when there is nothing more to say there is nothing more to say :)
This article is bad in general. It is like saying why do you want to drive a car rather than ride a horse. That is you can not answer such question out of context and as such the article is more click bait and useless. If you don't know when use polling, interrupt, or DMA for UART, then you a lot more education than such a simplistic view this article is trying to present.
I said enough in the previous statement. ISR driven serial comms are an example of a dated technique. They should only exist for cases where there is a requirement to wake a chip from a deep sleep. After which, the receiving device should transition to a DMA aggregate for both TX and RX data.
Quite frankly, I find these articles are mostly a tedious regurgitation of the obvious. I wonder whether they are generated by ChatGPT or something similar.
Sometimes we want to dump out some machine state to a UART when we encounter a hard fault or so. In these cases, we generally want to avoid using an interrupt or DMA driven UART in order to avoid the complexity which may not perform correctly during a hard fault condition. In these cases, we generally may want to consider using the UART in polled mode. Some UART controllers do not have a built-in FIFO, STM23 is an example of that. If you are going to run a non-FIFO-enabled UART in interrupt mode, then be aware that you will receive an interrupt request for every byte tx'd, and every byte rx'd. Consider your application's data rate, and number of UARTs in use. DMA can be an efficient alternative to interrupts, if it's available.
It depends on your architecture. Memory corruption can’t be possible if correctly designed. One thing must be remembered for embedded firmware specifically for microcontrollers, never allocate memory on fly. It has to be fixed at the time of design. If you want real time communication without any delay and background processing must be needed then use ISR , your protocol must have start of line , end of line and checksum, check every character till you get end of line character. Suppose if you don’t want to interrupt on every character then do a trick , specially when strict timing is not needed, start getting everything in circular buffer using ISR until no character receives after certain time ( reset timer when got a character ).