STM32 ADC is a pretty complex peripheral. It is designed to be flexible enough to accomplish complex tasks. We will dedicate a few posts where we will try to cover the main features and give working examples of code. The block schematic may look scary the first time, but it can be split into several pieces that are responsible for different functions if you look closer. Will will go through them step by step to make it look brighter.
This is a basic oscilloscope on STM32F103C8T6 microcontroller. The circuit board is kept very simple. No fancy analog stuff – just diode protection circuit and resistor divider. It is a two-channel 8-bit scope with 300kSps each. It accepts voltage levels from 0 to 6.6V. Data stream via USB is live and goes to PC GUI called mini scope v4.Here you have some basic controls, including sampling rate, triggering source and level, buffer size, and other handy stuff. Building a custom USB oscilloscope might seem like a fun project, it can be a lot of work and may not be worth the effort for most people. Here are a few reasons why: All that being said, if you’re really passionate about electronics and want to learn more about how USB oscilloscopes work, building your custom USB oscilloscope can be a gratifying project. Just be prepared for a lot of hard work and possibly some frustration along the way!
In the last part of the tutorial, we have covered simple USART routines that send data directly to USART peripheral. This is OK to use such an approach when a project isn’t time-critical and processing resources are far from limits. But most often, we stuck with these limiting factors, mainly when RTOS is used or when we perform necessary real-time data processing. And having USART routines with while the loop-based wait isn’t a good idea – it steals processing power only to send a data. As you may guess – next step is to employ interrupts. As you can see, there are many sources to trigger interrupts, and each of them is used for a different purpose. To use one or another interrupt, first, it has to be enabled in USART control register (USART_CR1, USART_CR2, or USART_CR3). Then NVIC USART_IRQn channel has to be enabled to map interrupt to its service routine. Because NVIC has only one vector for all USART interrupt triggers, service routine has to figure out which of interrupts has triggered an event. This is done by…
When we need some feedback from the microcontroller, usually we use USART. It allows to output messages and debug information to the terminal screen. Also, data can be sent to MCU same way. For this purpose, STM32 microcontrollers have more than one USART interface allowing to have multiple streams of data output and input. USART interface is designed to be very versatile, allowing to have lots of modes including LIN, IrDA, Smart card emulation, DMA based transmissions. But for now, let’s focus on standard USART communications we could send and receive messages from the terminal window.
Probably one of the key features of any microcontroller is the interrupt system. ARM Cortex-M3 microcontrollers may have up to 256 interrupted sources. The first 15 interrupt sources are called system exceptions. These exceptions arise within Cortex core like reset, NMI, hard fault and error, debug, and SystTick timer interrupt. In the exception table, they start from address 0x00000004 and are numbered from 1 to 15. There is no 0 number exception (FYI – the very top of exception table address is used to store the starting point of stack pointer): Each exception vector holds the four-byte address of the service routine that is called when an exception occurs. Exception table usually is located in startup code like this:
Previously we learned how to compile STM32VL Discovery projects that were included in the package. But to understand how to write our own programs, we need to get to some basics. I think the best place to start is the input and output system (I/O). Before we begin to write some code, let’s go through what’s inside STM32 ports. If you look into the STM32 reference manual, you’ll find that the I/O system is pretty flexible. Port pins can work in several modes: Input floating; Input pull-up; Input pull-down; Analog; Output open drain; Output push-pull; Alternate function push-pull; Alternate function open drain. Pins are organized as 16-bit ports that have their names like PORTA, PORTB, PORC, PORTD… Ports are 16-bit wide; they are controlled with 32-bit words. Individually each port pin can be configured to one of these functions. Additionally, each pin’s maximum speed can be set to one of the values: 2MHz, 10MHz, and 50MHz. STM32 I/Os are 5V tolerant. Anyway, the proper design should use 5 to 3.3V level converters.
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