Yesterday evening, I spent a long time troubleshooting the serial DMA issue on STM32, so this morning I decided to write a blog post to share my understanding of serial DMA transmission. In this article, I’ll discuss three key points:
1. What is a serial port DMA request?
2. A brief review of the serial port.
3. The process of serial DMA transmission.
**1. What is a Serial Port DMA Request (Battleship STM32 Development Board)?**
Before diving into this topic, let me briefly recall what DMA is. According to the PPT from the atomic brother:
DMA stands for Direct Memory Access, which allows data to be transferred directly between memory and peripherals without CPU intervention. Once the CPU initializes the transfer, the DMA controller handles the rest.
STM32 has two DMA controllers: DMA1 (with 7 channels) and DMA2 (with 5 channels, available only in high-capacity models). Each channel is responsible for handling requests from peripherals like TIMx, ADCx, SPIx, I2Cx, and USARTx. An arbiter manages the priority of these requests.
The main purpose of DMA is to reduce the CPU's workload. Instead of the CPU managing every single data transfer, DMA takes care of it autonomously.
Now, regarding the serial port DMA request: When you want to send or receive data via the serial port, the DMA can be used to handle the data transfer. For example, instead of the CPU writing data to the serial port manually, the DMA can copy data from memory to the serial port’s buffer automatically. This makes the process much faster and less resource-intensive for the CPU.
**2. Brief Review of the Serial Port (Library Function)**
In this section, I’ll briefly recap how the serial port works using the standard library functions. For instance, if you want to receive data from a PC through the serial port and then echo it back, you’d typically configure a receiving interrupt.
When data arrives at the serial port, an interrupt is triggered. Inside the interrupt service routine, you check whether the reception is complete. If it is, you set a flag and then, in the main loop, read the data from the buffer and send it back through the serial port.
This method works well for small amounts of data, but when dealing with larger volumes, using DMA becomes more efficient.
**3. Serial DMA Transmission Process (Library Function)**
Let’s take a look at a real example from the Battleship STM32 development board. The experiment involves sending data via USART1 using DMA, triggered by pressing KEY0. Each press initiates a DMA transfer, and the progress is displayed on the TFTLCD module.
Here’s how it works:
- First, we allocate a block of memory in SRAM (e.g., 5200 bytes) as the SendBuff array. This will hold the data to be sent via USART1.
- Then, we fill SendBuff with the data to be transmitted. The way the data is filled in is quite clever and worth learning.
- Next, we check if KEY0 is pressed. If it is, we enable the serial port’s DMA mode (by setting the DMAT bit in USART_CR3), and also start the DMA transfer by enabling DMA1 Channel 4.
- The DMA configuration includes setting the source and destination addresses, as well as the number of data items to transfer.
Once the DMA transfer starts, the data is moved from memory to the serial port automatically. The transfer time depends on the baud rate. For example, at 9600 bps, the transfer speed is about 1200 bytes per second. So, transferring 5200 bytes would take roughly 4 seconds.
Using DMA not only improves efficiency but also frees up the CPU to handle other tasks while the data is being transmitted. This makes it a powerful tool for real-time communication applications.
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