Microcontroller – Deniz Can Çığşar

If you are developing on Arm platform, you have extensive set of methods to establish link between host and target platform, such as:

Serial port
USB
Bluetooth and bluetooth low energy
ITM
Semihosting

The last two is the most easiest and convenient one. Because if you connect the debugger to the target system, you already have a device that can communicate, the cable, connector etc for the purpose.. You do not need a USB to TTL device, or complex USB firmware.. Actually with Art library it is not that complex, but that’s another topic.

With semihosting, the target platform, MCU can access to host resources via debug agent:
On the internet, there are clones of the same information on the subject. They tell linking against a library that supports semihosting however, there isn’t much about how the semihosting is implemented.

As far as I understand, semihosting is not implemented much on the targed side, but mostly implemented on the debugger software.. I am not %100 sure about this but looking at the how it works, I only see a “trigger” on target side, the rest is cared out automatically.. For instance the following code debugged on LPCXpresso pushed two lines seen in the “Console” tab at the bottom:

The first line “test 0” written by printf function, the second line with “write” text is written by _write function.. The debugger carries the texts into the console embedded into the eclipse.. _write is mentioned in the semihosting documentation but not defined explicitly. By walking through the source code of newlib, I extracted the implementation. The simplest form of it (dropping the checks etc is the following:

My purpose was to write the semihosting for my Art library that works on multiplatform but this time for STM32F429 series MCU. For those who is not familiar with GCC Assembly, the compiled code is the following:
What is does is simple.. It loads r0 with r2 which is loaded with 5 before; r1 with r3, which is loaded with address of the argument table before; and executes breakpoint instruction with code 0x00ab. In manual bkpt instruction is defined as following:

The BKPT instruction causes the processor to enter Debug state. Debug tools can use this to investigate system state when the instruction at a particular address is reached.

imm is ignored by the processor. If required, a debugger can use it to store additional information about the breakpoint.

From this, I deduce that bkpt stops the CPU, informs the debugger. Debugger reads the code given with bkpt instruction, and takes action. In semihosting page of Arm the format is described as following:

BKPT 0xAB

For ARMv6-M and ARMv7-M, Thumb state only.

So, debugger reads the code supplied with bkpt instruction, if it sees 0xAB than it looks at R0. R0 tells the semihosting operation. Those are:

0x01: SYS_OPEN
0x02: SYS_CLOSE
0x05: SYS_WRITE
0x06: SYS_READ
0x11: SYS_TIME
0x12: SYS_SYSTEM

There are more commands, you can follow the Arm’s page.. Here, our semihostWrite function used 0x05, which is SYS_WRITE. R1 points to arrray that holds the arguments used by the command. You can extract the contents of the array in the semihostWrite function. But to name those, there are 3 words (32 bit unsigned integers):

File number: 1
The file handles are the numbers, and file 1 looks like belonging to debug console.. I think 0x01 SYS_OPEN will open other files stored in the system, and will return numbers other than 1 as handle.
An address to the buffer to read by debugger.
Length of the buffer to read.

The debugger does the rest. Reads the buffer pointed in the second element in the array, with the length in the third element of the array, copies the contents of the buffer to the terminal..

The opportunity to read/write files stored in the host system is interesting. That way you can dump large amount of data into/from the device without other means of protocol, media etc. Commands 0x11 and 0x12 are also unique. With 0x11 SYS_TIME, you can read the system time and date. With 0x12 SYS_SYSTEM (Arm is really bad at naming) you can execute commands on the system..

Semihosting is very nice but has one bad drawback: It takes a precious time to execute and in that time the CPU is stalled.. I run the following code to test the time it requires to transfer a simple string:

This write requires whopping 140ms with ST-Link Debugger interfaced with OpenOCD. In the following oscilloscope view of the process, in the blue areas the led is toggled, between those areas the debugger is in control, so the cpu is stalled:

Transfer of a longer string does not effect much the stalling time. I suspected that the handling is time consuming, not the transfer.. May be the debugger is not event driven, but pooling driven.. Who knows?

I do not know if this is because of ST-Link or OpenOCD but knowing that within this period CPU is stalled is not except-able in many applications. I am sure there will be use cases when the time is not critical, for instance when the system is not doing time sensitive operations. At boot time some data can be read from the host system, or at certain times some data can be pushed to the host however, it would not be a regular, non intrusive debugging alternative.. What an opportunity lost..

I’ll implement the semihosting classes anyway. Because most of the handling is done by the debugger it will be very lightweight and there will be gains. However, I’ll use it sparingly..

The sample Art framework code that reads data from host would be like this:

HostStream stream;
stream.open("~/SampleData.dat");
stream.read(data, sizeof(data));
stream.close();

Reading time and date at the host:

Time time = Time::hostTime();
Date date = Date::hostDate();

For instance, you may set the Rtc date and time by the following code:

rtc()->open();
rtc()->setDateTime(DateTime::hostDateTime());

I wish it were faster..

I did it again. I went in the different direction from “common sense” and used dynamic memory allocation in my embedded products. Many expert embedded programmers would say “No, you should not”, however if you implemented a correct allocator why not?

The major concerns about dynamic memory in microcontroller is:

The allocation/deallocation time is unpredictable.
Memory fragmentation that leads unusable blocks.
The memory requirement is unpredictable.

Scary? Yes it is.. But, if the given bottlenecks were fixed, dynamic allocation would make developers life a lot easier. Before delving into how to solve those problems, lets have a look at what dynamic allocation offers us:

String class

First of all, a string class that handles the string and being able to write the following would be very nice:

String myDearString = "My Dear String";
String myOtherString = myDearString + " makes my life easier";
if (myOtherString.startsWith(myDearString))
  ..

I’ll write details of my String class in another post. To be fair, I just want to say myDearString is not allocated in heap, because string literals are kept in ROM, String class exploits that by looking at the address of argument, and just keeps a pointer to it.

myOtherString is a sum of two strings (actually two literal strings). The class allocates enough RAM, and combines the chunks, leading “My Dear String makes my life easier”.

Message Queues

In Awin library (a GUI library), many messages are passed to application: Keyboard presses, releases, mouse movements etc. Those are different types with nearly similar sizes. If I didn’t used heap, I should create memory pools for those. Another layer of burden..

Dynamic Data

Again in Awin library, the clipping regions are calculated on the fly. For those haven’t cared what clipping region is: Suppose the following, a button inside window:

Window manager has two options to draw whole window: First draws Window area, which erases Button area, than draws Button area. This causes two problems: Unnecessary drawing the area under the Button, and unnecessary flicker. If you don’t count it as an effect though.

The window manager splits the area into areas that don’t have intersections as following:

Than calls draw methods of each class with given regions.. Because the draw methods do not erase what other methods have done, this method save CPU time (read it as speed, less power, longer battery time) and flicker at a certain leve as well.

Suppose the application made another objects, let say, a label visible. Now the clipping region map would be different..

To support dynamic (did I say dynamic) mapping of clipping regions, I would have created another memory pool for clipping subsystem. Which is managing another memory.

Zero Configuration

For years, when I had to use the code other people created for embedded systems, such as Bluetooth LE, FAT, protocol handling libraries, I had to deal with ugly macros to start up. To turn on this feature write this macro, to set buffer size, define this macro, to define how many tasks your application would use set this macro.

Because it creates a lot of errors, Keil made a configuration page for its macros:

I cannot blame that is bad. In the past I used it to configure my builds to turn on and off features. However, it is just a neat way to define macros that’s it..

What if the system configures it self and gives the developer a better interface. For instance to set a serial ports transmit fifo, wouldn’t be more readable if we write:

serialPort.setTxFifoSize(1024);

Or to create a new thread:

Thread thread;
thread.start(myThreadFunction);

It creates a thread with default size of stack. You may set the stack size as well:

Thread thread;
thread.setStackSize(512);
thread.start(myThreadFunction);

I know, those last two samples are static allocations that uses dynamic allocation. But neat! Isn’t it?

Deniz Can Çığşar

Tag: Microcontroller

What is semihosting and why it is heartbreaking

I use heap on microcontrollers, am I doomed? (1)

String class

Message Queues

Dynamic Data

Zero Configuration