STM32 Serial Communication

STM32 micros just like any other micro provide hardware for serial communication. As we all know serial communication is a very important tool for debugging, connecting with external hardware like RFID, GPS, GSM modems, etc. and for performing other communication-related tasks. STM32s have several hardware serial (USART) ports. The number of ports available in a STM32 micro is dependent on device family type and the device itself. Typically there are at least 5 serial ports. Apart from the basic serial communication needs, STM32’s USART hardware also have support for LIN, one-wire, smart card protocols, SPI and IrDA. We can even utilize STM32’s DMA controller with USART hardware to achieve high speed automated data communication. Thus these hardware are truly universal synchronous-asynchronous receiver-transmitters.

In any standard serial communication, we need three wires – TX, RX and GND. TX pin is an output pin and transmits data serially to another device’s RX pin. RX pin is an input pin and is responsible for receiving data from another device’s TX pin. The two devices connected in this way must have same ground (GND). There are other pins like CTS and RTS which are used for hardware flow control. Additionally there’s also another pin called CK. It is transmitter’s clock output and used usually in SPI and other modes.

Depending on package, USART pins are arranged in the following pattern:

D9 67 29 41 PA8 USART1_CK
C10 70 32 44 PA11 USART1_CTS
B10 71 33 45 PA12 USART1_RTS
C9 68 30 42 PA9 USART1_TX
D10 69 31 43 PA10 USART1_RX
G3 29 14 20 PA4 USART2_CK
G2 23 10 14 PA0 USART2_CTS
H2 24 11 15 PA1 USART2_RTS
J2 25 12 16 PA2 USART2_TX
K2 26 13 17 PA3 USART2_RX
K8 51 25 33 PB12 USART3_CK
J8 52 26 34 PB13 USART3_CTS
H8 53 27 35 PB14 USART3_RTS
J7 47 21 29 PB10 USART3_TX
K7 48 22 30 PB11 USART3_RX
B8 79 - 52 PC11 UART4_RX
B9 78 - 51 PC10 UART4_TX
B7 83 54 PD2 UART5_RX
C8 80 - 53 PC12 UART5_TX

Personally I’m interested in LQFP packages, particularly 48 and 64 packages as they are mostly used in the most common STM32 development boards. I suggest locating USART/UART pins before working with them.

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Porting ST Standard Peripheral Library (SPL) with MikroC PRO for ARM

What is this Standard Peripheral Library (SPL)?

It’s just a collection of hardware libraries that provide an easy approach to any STM32 ARM programmer. It has support for every peripheral a STM32 micro has like CAN, USB, ADC, Timers, etc. In short it’s a hardware abstraction layer fully covering the STM32.

Why use the ST SPL?

  1. Reduce coding time by spending less time figuring out 32 bit register values and going through a near thousand page reference manual.
  2. Take the advantages of the built-in code library of MikroC which other compilers don’t offer.
  3. SPL is used by most STM32 ARM programmers and so it is a widely used tool with a large community.
  4. Most books and documents on STM32 ARMs are based on SPL.
  5. A compiler like MikroC doesn’t give full focus towards STM32. It has other ARM concerns like TI’s TIVA, NXP, etc and so MikroC’s built-in libraries are not 100% compatible with all ST hardware.
  6. Following the previous point MikroC (at least at the time of writing this article) doesn’t give library support for hardware like the DMA and so there’s incompleteness to some extent. It is also worth noting that depending on MikroC’s library also has the disadvantage of code size and efficiency. It is in general true for every compiler. However MikroC’s IDE is cool enough to do things with ease and that’s why I like it. Unless you are a Maple or Arduino fanboy, you’ll get my point.

Software Needed

  1. Modified ST Standard Peripheral Library (SPL) as supplied here: STM32 Standard Peripheral Library for MikroC. It works for STM32F10x micros only.
  2. MikroC PRO for ARM compiler.
  3. Flash Loader Demo or ST-Link Programmer Software.

Hardware Needed

  1. A STM32F10x Development Board.
  2. A USB-Serial Converter or ST-Link Programmer.
  3. Some basic components like a mini bread board, connecting wires/jumpers, a LED, a 220Ω 0.25W resistor and a push button.
  4. Optional debugger hardware.


How to use the Standard Peripheral Library?

First we need to prepare our MikroC PRO for ARM compiler for linking with SPL. I’m assuming that the compiler is preinstalled. First go to the compiler’s installation folder and look for the include folder. In my case, it is:
C:\Users\Public\Documents\Mikroelektronika\mikroC PRO for ARM\Include.

In this folder there’s a file named stdint.h. It’s basically a definition of variable types and other related stuffs. Rename it to stdint (backup).h and copy the new stdint.h file from the supplied folder to this location. We are done here.

stdint modification

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XMega Clock System

Okay firstly the reason I wrote about the clock system instead of I/O ports or something else in this second post of the XMega series is simply because of the fact that without understanding clock configurations you won’t get what you want from your chip. Since XMega’s clock system is software-level configurable and complex at first, it makes itself the first priority module before anything else.

There are several clock sources that XMega micros can use as clock. These are both internal and external clock sources. As stated earlier, clocks are not set by fuse settings unlike traditional Mega AVRs, they are controlled by software and the outputs from these sources can be multiplied using the internal Phase Locked-Loop (PLL). Thus there’s a wide range of clocks available in the XMega and it’ll not be needed to shut down and reprogram the XMega just to change its clock – a great relief from meddling with fuse settings. Some port pins can also output the system clock frequency. Literally there’s no need to depend on external quartz crystals which are not available in all frequencies.

XMega Clock System Internal
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