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Category Archives: AVR32 4 Dummies

AVR32 Interrupt System and Wrap up

It’s been a while since the last post, this is the last post on the AVR32 4 dummies series because after this one there isn’t nothing to talk about. I decided not to talk about the UC3 peripherals because is standard stuff, UART, SPI, I2C, about the device drivers construction, the reason is the same there are thousands of ways to do it, and I still didn’t found the ideal way to do them.

MOS Interrupt Handling and interrupt thread switch.

In the previous post, I talked about the different ways that MOS has to switch threads, one of them was via interrupt, to do that MOS vector the interrupts via software, by doing this it looses precious time attending interrupt, because every time that an interrupt is signalized MOS  code runs to find out what interrupt it was and if is time to switch threads.

This indirect way to call an interrupt service routine makes the code to configure them portable, meaning that I didn’t touch my Kernel code between architectures, that may seem obvious but I’m very proud of that XD.

Going back to the interrupt thread switch, the MOS common trap (when an interrupt is signaled the CPU jumps to this function) is responsible to arrange things to MOS interrupt handling system. That arrangement is as simple as passing a pointer to pointer to the thread context.  As I said before, the context switch routine and the common trap are directly connected, they store the CPU registers and flags by the same order (so that when a thread is switched back all registers are popped the right way).

Although the common trap is architecture dependent the system common trap is not (it’s writted in C), so the purpose of the common trap is to store the all the registers not saved by the CPU and call the system common trap.
The system common trap simply delegates to the (MOS) interrupt controller the handling of the interrupt. In the middle of the interrupt handling the interrupt controller queries the kernel to check if a thread switch is needed. Remember the pointer to pointer that the common trap saves? The context switch happens by changing the value pointed (by the pointer xDD) to the next thread stack. By doing this when the common trap returns from the interrupt, it will restore not the thread that was running before the ISR but a new thread (how not confusing is this xD).

You can found on github the system common trap and the common trap   (labelled as int0)

UC3 Interrupt Controller

I didn’t make my own irq controller (shame on me), I browsed the Atmel peripherals library and take out the sources to the MOS. The main reason for that was the initialization stuff, and the fact that in UC3 the mask and unmask operations are done directly on the peripheral not on the interrupt controller. So there isn’t much to do, just initialization, ISR configuration and non vectored interrupt discovery.

As I said in one of the previous posts, the AVR32 architecture doesn’t have fixed memory positions for the exceptions/interrupts (being the reset exception the exception xDDD), they all have an indirect location. That locations is given by the Exception Vector Base Address (EVBA). The interrupt handlers and exception handlers have their location fixed in relation of the EVBA value. By having this kind of mechanism you can virtually have your handlers in any location that you want, that is a cool feature of this architecture. So when making your own interrupt controller driver don’t forget to set the evba correctly 😉

Wrap Up

First I apologize for the lack of AVR32 content on this post, the reality is that there isn’t much to talk about about interrupt handling on AVR32, is very simple to understand. I learn a precious thing by not making any kind of device driver, learn how to read and understand other people code, really usually I just go and make everything from the bit on register X to the pretty printf that writes on uart. In embedded world that is an interesting skill to achieve, since in the real world you don’t make all device drivers, you use what you have and most of the time, those were made by people that don’t understand how software is supposed to be done.

I had fun with AVR32, didn’t really like some stuff, like the fact that you can’t mask and unmask specific peripheral interrupts in one place (in the UC3, this doesn’t mean that all CPUs work this way), I liked the many modes of interrupt and the fact that you can individually disable them, but honestly in real world you don’t use just 4 interrupts, you want to map all of them and not care about the interrupt controller.

Finally I’m sad about not liking this architecture, the main reason is the development environment, because all that I tried to this point were horrible, and AVR studio was really a breath of fresh air and astonishing way to program to embedded systems.

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AVR32 First Contact, First Code

So this is the third of the AVR32 4 dummies post series, you can check the other blog posts on:

I finally started coding some stuff, namely the board startup, the context switch, the interrupt controller driver and the system timer driver. Along the way I discovered more things about the architecture, namely where the AVR32 call convention is documented. The Atmel support, when I contacted them to know about the call convention, forwarded me to the IAR compiler Reference Guide. I loled and checked if my reverse engineering discovers were right (they were).

Before any more comments and discoveries, here it is the overall description on my development board and CPU:

Board: EVK1104:

  • CPU : AT32UC3A3256, AVR32.
    • RAM: 128Kb.
    • ROM: 256Kb.
    • SPI: 6.
    • UART: 4.
    • I2C: 2.
    • ADC: 10bits (max resolution), with 8 channels, up to 384 ksps.
    • Speed: up to 66MHz.
  • 16 bit DAC.
  • SD card.
  • 256 Mbyte external RAM.
  • 256 Mbyte external ROM.
  • Touch Wheel for QMatrix.
  • 240×320 RBG LCD

Both the board and CPU websites are linked in the previous list, for more information about the EVK1104, download the Schematics and BOM since there is no user manual. The BOM (Bill of materials) have all the hardware used to create a dev board, so it’s the first place where you should go to find out what kind of hardware is “under the hood” or if you want a specific datasheet.

AVR32 Bootloader

A bootloader is a simple program or piece of code that initializes the hardware so that an operating system or application can run. The bootloader can be “called” by the reset event handler or, as usual, be part of the reset event handler. On AVR32 architecture the reset event is the only event that is statically mapped on the memory layout, at the address 0x80000000.

The bootloader present in the Atmel software libraries for my dev board follows the same principles on the bootloaders that I’ve done before for the ARM architecture:

  1. Set stack pointer.
  2. Set other system required configurations.
  3. Load the .data section (initialized variables).
  4. Set the .bss section to 0(uninitialized variables).
  5. Load .text to RAM (optional, used for better performance)
  6. Run the the global instances constructors (c++ only).
  7. Call main (or a operating system boot)

The loads are simple memcpy, from one place to another, in most of the cases from Flash to RAM.

I didn’t do my own bootloader for the board, I used one of the Atmel startups and added what I needed, namely the constructors call (MOS is in C++). I did the same for the linker script, I grabbed the Atmel ldscript and corrected some errors on it, again because of my C++ code (on the default linker scripts, although the C++ init arrays are present, the global variables are misplaced and without any change the init and fini addresses are always the same, for more information about C++ initialization check out the previous link of IAR compiler reference).

You can check out the startup and the ldscript via github.

Context Switch

The context switch is the operating system routine responsible for changing threads, this is done by saving the context of the current thread and loading the context of the next thread. A thread’s context is the minimal information needed to represent a thread’s execution state at the time of the switch. The context should be sufficient so that when a thread is switched its previous execution state can be restored and the thread execution continues like nothing happened.
In CPUs without MMU, FPU, etc, the thread context is reduced to the CPU registers, in AVR32 from R0-R15.

MOS has two ways to switch threads:

  • By interrupt: when an interrupt occurs the system checks if it is time to switch threads (MOS have a time-slice scheduler), if so the system changes the current thread context pointer (this context is stored on the epilogue of the interrupt and passed to system interrupt service routine) to the next thread context. With this way the context switch only happens when the interrupt returns.
  • By thread choice: when a thread doesn’t have nothing to do or is waiting to be signalized it calls kernel primitives like Yield, Sleep or Wait (in synchronizers), this functions can trigger a context switch.

Because of this feature, the two context switch implementations have to be linked to each other. Since the AVR32 interrupt handling automatically stores the r8-r12 registers, the “normal” context switch must save this registers first by the same order that is done on the interrupt. After that it’s needed to save the rest of the registers r0-r7, lr and pc. You’re probably asking why the sp (stack pointer) isn’t saved, the stack pointer is the only pointer that MOS has to know where the thread context is, so the stack pointer is saved on the thread instance, so that further switches can restore the thread context.

I’ve already pushed the AVR32 thread context and the context switch to github, go check it out.

If you check the context switch code, you will see that I don’t store all registers. Actually what I’m doing in some cases is to move stack pointer further down. For instance when the “normal” context switch is called, the caller must preserve all the scratch registers and since the context switch doesn’t use any local variable, I don’t need to store them (they are already stored in the thread stack by the caller of the context switch) so I move the stack  pointer down, saving precious memory accesses.

But when I restore the next thread I restore all the registers, the main reason is because it’s “impossible” to know how the next thread switched (by interrupt or by the normal way) so I simply restore them all.

On another note, the status register is also saved on the context switch, again this isn’t needed when thread is switched using the “normal” way, but when switched via interrupt is mandatory.

Final Comments

When doing the context switch I felt a bit forced to do things the AVR32 way, since the interrupt handling system stores automatically R8-R12 registers into the system stack. So MOS port will not take advantage of the Application mode, and all the code is going to run on System mode. After browsing the Atmel ldscript I felt a little disappointing on the development leftovers that were present and the fact that they don’t even bother to test the CPU that I’m using with C++, I can say this because the global constructors pointers are not correctly configured on the ldscript.
Also, In a previous post I hysterically talked about the status register being mapped on memory, I was wrong, you need to use special assembly instructions to read and write things to the SR.

On the next post I’ll talk about the interrupt handling system, the UC3 peripherals and the device drivers construction.

Feel free to ask any question or comment, see you next time.

Micro Operating System Overview, Port to AVR32

In this post I’m going to talk about the operating system that I made with the help of Sorcha. This post is the second on the AVR32 for dummies post series.

So the Micro Operating System or MOS is a simple, source readable operating system, or I believe so xD.

MOS is in C++ and divided in three main layers, System, Kernel and HAL (Hardware Abstraction Layer):

The System layer is the operating system development API, just like any operating system this layer is “responsible” for the application portability, by having all the possible software and hardware abstraction. This description of the System layer is my future goal, today this layer has little code base and little device drivers support.

The Kernel layer is where all my operating system knowledge is into. The kernel is based in two infra-structures that I studied before, eCos and user-threads.
The Kernel layer is divided in three sub-layers, InterruptController, Threading and Concurrent.

  • The Concurrent sub-layer is where all the synchronization primitives are implemented, things like atomic operations and synchronizers.
  • The Threading sub-layer is responsible for the Threads handling, it contains only two classes UThread and UScheduler.
  • The InterruptController is where the system delegate interrupt handling and treatment.

The HAL layer is the only one that knows what kind of hardware the system is running on. This layer is yet divided by two sub-layers, Target and Architecture Abstraction.

  • The Target Abstraction, is the only software component that knows on witch specific hardware the system is running on, so this sub-layer is responsible for all the hardware configuration and management.
  • The Architecture Abstraction is the sub-layer that knows on what kind of architecture the system is running. This sub-layer implements all system operations that it can, things like enable and disable interrupts, boot loaders, interrupt traps and the context switch.

Today MOS only supports ARM7 architecture. There is some LPC2xxx device drivers on it, and only ran on a development board the yaab2294 (LPC2294 with 8mb ram/rom expansion).

But that is about to change, with this port to AVR32 the operating system will be in two different architectures, processors and boards \o/.

The only thing I need to do is port the HAL layer to the AVR32 architecture and to my board (EVK1104, UC3 processor). In the next posts I’m going to talk more specifically on what is needed to do to port mos to another architecture and board.

MOS doesn’t have a pretty programming interface yet, but where’s some of it’s features:

  • Clean code, based on managed languages conventions (mainly C#) so that managed programmers don’t run away when they see the first code lines.
  • The synchronization don’t disable interrupts, that’s a cool feature made with a global system lock (a counter) that prevents interrupt treatment to change system global variables or switch threads.
  • Delegation on interrupt service routines, when an interrupt treatment is heavy, it’s possible to delegate that work to system defined thread, by doing so, the system is “always” susceptible to interrupts.

Final Comments

The MOS source code is available on github and I’ve created a new branch for the AVR32 port, in here.
This operating system is my way to apply all the concepts that I learned studying embedded and PC OSs and migrate my software design skills from managed to unmanaged.

AVR32 First Discoveries

I started my study on AVR32 architecture, I’m amazed, angry and confused on what I discovered in this short period of time, the typical when your accommodate to awesome ARM architecture.

The development environment for the AVR32 is the infamous AVR Studio 5.0, based on Visual Studio (probably the best IDE that I’ve ever worked with) and have some cool and annoying features. The best feature of AVR Studio is the Visual Assist X plugin, basically that’s the best IntelliSense that I’ve ever used for C/C++ development. The most ridiculous thing that AVR Studio 5 have is claiming to have C++ support because that’s a total lie, when you download there is no C++ support, you have to download a buggy plugin if you want the support.

A cool feature of this IDE is the “Eclipse Like” toolchain settings editor, the problem with this? They removed the option to define the linker script (To those that don’t know, when you don’t define a linker script -T myscript.ld, the linker will use the toolchain defined script), fortunately if you configure in the “other options” the -T switch  with your linker script, it will be used instead of the default.

Nevertheless the AVR Studio 5 IDE seams a very powerful tool if you want build applications for the Atmel boards, but if you want to go deeper you will find some ridiculous bugs and workarounds.

AVR32 Quick Reference for Developers

The AVR32 architecture is divided in two main micro architectures, the AVR32A and the AVR32B. The AVR32A was designed to be cheap, by doing so this microarchitecture doesn’t have a full register bank for interrupt/event treatment. The AVR32B was designed to time sensitive applications, so the interrupts have their own register bank.

NOTE: Throughout this posts I’m only going to talk about AVR32A because the CPU I have is a AT32UC3A3256, AVR32A compatible with floating-point, so every time I refer AVR32 features and explanations I’m talking about AVR32A (for instance the AVR32B boot-loader and memory layout is different from the AVR32A).

Registers

There are 15 registers in this architecture they are:

  • R0-R8, general purpose registers.
  • R9-R12, argument registers.
  • R13 – The stack pointer.
  • R14 – The link register.
  • R15 – The program counter

You are probably asking where are the flags and other runtime information, I’ll talk about it later on this blog.

Call Convention

The AVR32 call convention is the follow:

  • R9 to R12 is used for arguments where R12 is the first argument and the R9 the forth. All other arguments must be passed by stack.
  • r8 to R12 are scratch registers.
  • R0 to R7 are preserved registers.
  • R12 is used as the return value.
  • R13 to r15 “architecture hardware” registers.

Processor Modes

AVR32 has 8 different working modes:

  •  Non Maskable interrupt – system events that can’t be ignored, this is design to events like TLB misses.
  •  Exception – the usual CPU exceptions, data abort, bad instruction,etc.
  •  Interrupt 3-0 – four different interrupt modes, being the interrupt 3 the most priority and interrupt 0 the least.
  •  Supervisor – the system mode, design to support operating systems.
  •  Application – the user mode, the only one that don’t have all the running permissions

On AVR32 when an interrupt/event is detected by the CPU, they store R8-R12 automatically leaving plenty of registers to  handle the interrupt/event.

Status Register

The status register is divided in two parts. In the lower halfword are the common execution flags (Carry, Zero, Overflow, etc). In the highest halfword are information about the mode, the interrupt availability and masks. The status register even have a scratch bit for the application to use if needed, that’s a funny feature. For more information check the Architecture Document at Atmel. Throughout the future blog posts I probably will talk about some specific flags that I’m going to use.

Remarks

  • The stack pointer (R13) is shadowed between Application mode and the system modes (all the other modes).
  • The Status Register is mapped directly on the memory at 0x0.
  • When an event/interrupt is attended the program counter points directly to the current instruction (That’s not true in all architectures, for instance in ARM the PC is one instruction ahead).
  • One cool feature of the AVR32 event treatment is the indirect exception vector, with that you can define your handler where you want (In ARM this handlers have a “fixed” memory location).

Final Comments

The AVR32 documentation is just horrible, there is little information about the “behind the scenes” I had to reverse engineer the call convention and that’s just ridiculous. If they are trying to beat ARM, they must explain what they were doing, they must get the attention of people who want to work with the hardware as much the people who make the applications to run on it.

I see interesting stuff, like the system registers, the four different interrupt modes and the development environment(Visual Studio + Visual Assist X = FTW) but I don’t feel that they care a lot about the developers, they don’t care about the people that want to work just above the hardware, “talking” to it and configuring it. From an application designer view, AVR32 is awesome, they give you all the device drivers and interfaces to talk to the hardware (something that is on going on the new ARM Cortex convention) and with Visual Studio the only difference between coding for PC or AVR32 is that USB JTAG cable connected to your PC.

References

AVR32 Architecture Manual

32-bit AVR UC3 Application Note

AVR32 for Dummies

So, me and Sorcha have ended our graduation project \o/.
You can browse the source code on github.

In July we submitted our project into a company contest, the Bithium Design Challenge.
The contest is divided in two parts, the concept part that we already won, the prize was an EVK1004, an AVR32 development board, the second part is where we need to implement the concept that we purposed.

The deadline is 7 October, and we need to start doing it XDD.

I started to study the AVR architecture and discover that there isn’t anything like the ARM documentation (we programmers must love documentation), so I decided to do some blog posts about where to start.

The project that I’m going to do is a simple Operating System port, one that we made (Micro Operating System), so the main bullets that I’m going to talk are:

– The AVR32 development environment.
– The AVR32 boot.
– Using AVR32 interrupt system.
– Making the context switch and atomic operations.

The bullets are not in order, I’m going to talk by the order that I learn those topics in the next 7 days.

Finally when the project is over I’m going to compare AVR32 with ARM7.

See you next time.

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