Lesson 6: Part 1: USART - Transmitting Data Out To PC

To expand the capability of the embedded system, a communication channel is always require. For example, to communicate with a PC, usually a Serial Port (RS232) will be use. Machine to machine communication usually is realise using RS232 or RS485/RS422 (longer distance). In embedded system, the peripheral that incharge of such function is UART(Universal asynchronous receiver transmitter) or USART (Universal synchronous asynchronous receiver transmitter)

As Debugging Tool

One useful usage of USART is in debugging. But inserting code that print debugging message, we can always know what is the value of variable, which part of the code MCU is running. Sometimes, it is more efficient by looking at the debug message instead of stepping through the program.

The down side of this is that a dedicated buffer has to be allocated to keep the debugging buffer(as sending USART speed is slow).

Physical Connection

USB To Serial Port < --> Max232 <--> USART/UART

In this tutorial, we will be looking at enabling communication between STM32F030 with PC through RS232. We will be enabling USART1 on MCU part. To enable communication with PC, an additional voltage converter(MAX232 chipset) circuit is require as shown below:

RS232 Circuit

The above image is take from here. It also include a good introduction of interfacing with PC.

Currently most of the PC/laptop no longer come with DB-9 Serial Port. And a replacement of this would be a USB to Serial converter as shown below:

USB to Serial Converter

USB To TTL

The easiest converter to use is shown below, which enabling direct interface into STM32F030, without requiring MAX232 as shown below:

USB to TTL
Enabling direct connection to STM32F030 USART

STM32F030 SFR Configurations

We will be using the following settings on STM32F030:

• Baud Rate: 115200bps
• Data bits: 8
• Stop bits: 1
• Parity : None
• Handshake : None

On PC, you can use teraterm, hyperterminal or similar software to open the Serial Port and accept data send from STM32F030. The above settings must be configure in software to ensure proper communication between PC and STM board.

As for the SFR section, it would be too lengthy to go through each of the register. I have heavily comment on the source code instead to provide explanation of each SFR. Refer here for the source code.

Source Code Explained

Below are the key actions implemented in source code to enable data transmission.

1. SFR configurations

• Enable GPIO block that contain USART TX/RX pn
•  Configure TX/RX pin to use as USART (so that is will not use as general purpose input/output)
• Enable USART block
• USART settings (baud rate, data bits, stop bits, parity, handshake)
• Enable USART function, and transmit, receive function

2.  Data Transmit Function (call by application)

• Putting first byte into transmit buffer
• Waiting first byte transmit complete
• Putting subsequenct byte into transmit buffer and wait until byte is transmit out. Repeating this until all bytes has been transmit

The example will be continuing printing out message 'Hello World!" through USART1.

Below is the image of my connection, using an USB to TTL device, directly connect to STM32

• White cable: 3V
• Blue cable: GND
• Grey cable: PA9, USART_TX
• Purple cable: PA10:USART_RX

Physical Connection

Source Code Implementation

The above source code is implementing a blocking transmit. Which means that the MCU will be waiting(a while loop) when current chracter being transmit out. Thus when sending full length of 'Hello World!', no other process can be executed (except ISR). This is inefficient but this is the simpliest way to ensure character transmitting is working.

Apart from blocking transmit, we can also perform USART transmit using:

• Interrupt: allow interrupt occur when current data has been transfer to transmit buffer(it would take some time to transmit out), so we can load next data into TDR. This is better than blocking, but still require ISR code to loadbuffer into TDR
• DMA: enabling DMA auto transfer when current data has been transfer to transmit buffer. This is the most efficient method, but also more complex as it involve interrupt + DMA control.

Later stage we shall be looking for DMA method. For now, we should get things work in the simpliest method: blocking transmit/receive.

Lastly, I attached a short video on my capture on data transmit from STM32 on this tutorial.