Use basic run/stop debug

Before you begin

To follow the instructions in this Learning Path, clone or download the Blinky example project repo on the same machine where you have installed Arm Keil MDK and the Corstone-300 Ecosystem FVP. Double-click the Blinky.Debug+AVH.uvprojx file to open it in µVision.

Hardware Breakpoints

To use hardware breakpoints in µVision follow the steps outlined below:

1. Build (F7) the example Blinky project you opened.
2. Start a Debug Session (Ctrl+F5) to enter the µVision debugger.
3. Run (F5) the application.
4. Bring Blinky.c in focus by clicking on its tab. If it is not visible, double-click on it in the Project window.
5. In Blinky.c, scroll down to the while (1) loop in the main function near line 42 as shown here:
   
6. Note the darker grey blocks on the left of the line numbers. They indicate assembly code being present and that you can set a hardware breakpoint on these lines. You can also see these blocks in the Disassembly window:
   
7. While the program is still running, click to the left of a suitable line in the while loop (for example at line 47) and a red circle will be created. This is a hardware breakpoint. The simulated Arm Cortex-M55 has eight hardware breakpoints. μVision will warn you if you exceed this limit.
8. The program will soon stop at the line where you have set the breakpoint on as shown below. The yellow arrow is the current program counter position. This will be the next instruction executed. The cyan arrow is a placeholder you can use to explore the relationship between a source window and the Disassembly window.
   
9. Add another breakpoint in while (1) (for example at line 50).
10. Each time you click on Run (F5), the program will cycle to the next breakpoint.

Manage Breakpoints

1. Go to Debug - Breakpoints (Ctrl-B) to manage breakpoints:
   
2. You can temporarily unselect, delete or create breakpoints in this window. It is easier to create a breakpoint by clicking in a source file or the Disassembly window.
3. Watchpoints are also created in this window.
4. Select Kill All to remove all breakpoints.
5. Click Close.
6. Continue to Run (F5) for the next exercise.

Call Stack + Locals Window

The Call Stack + Locals window displays call stack contents as well as any local variables belonging to the active function. If possible, the values of the local variables will be displayed. If this is not possible, a message such as <not in scope> will be displayed. The Call Stack + Locals window visibility can be toggled by selecting View - Call Stack window.

Call Stack Use Case

When the program is stopped, the list of stacked functions is displayed. This is when you need to know which functions have been called and are stored on the stack. This is useful to find program flow problems such as crashes. Normal functions are displayed only when they are on the stack. They are removed and added as appropriate.

1. Stop the program with the Stop icon. The program will probably stop in the Delay function.
2. Click on the Call Stack + Locals window in the bottom right corner of μVision.
3. Inspect the various entries in the Call Stack + Locals window as shown below in this simple example. Local variables are displayed only when they are in scope:
   
4. Set a breakpoint in the while (1) loop at line 50 on the g_ledSet = 1;.
5. Run (F5) the application.
6. Shortly after, the program will stop there.
7. Step (F11) with Blinky.c in focus (its name in its tab is underlined).
8. Note how the variables displayed change in the Call Stack + Locals window.
9. Step (F11) a few more times.
10. Right-click on a function and select either Show Caller Code or Show Callee Code and this will be highlighted in the Disassembly and source code windows:
    
11. Step Out (Ctrl+F11) to exit a function immediately.
12. Remove the Breakpoint (by clicking on its red circle ) to continue.

Watch and Memory Windows

The Watch and Memory windows display variable values in real-time using Arm CoreSight technology. It is also possible to put or insert values into these memory locations in real-time using the Memory window while the program is running. You can change memory in a Watch window if data is not changing fast.

There is a global variable g_msTicks located in Blinky.c near line 11 that you can use in the Watch and Memory windows.

Watch Window

1. Run (F5) the application.
2. Right click on g_msTicks in Blinky.c near line 11 and select Add ‘g_msTicks’ to… and select Watch 1.
3. Go to View in the main menu and enable Periodic Window Update:
   
4. g_msTicks will be displayed in Watch 1:
   
5. The values of g_msTicks are updated in real-time if Periodic Window Update is enabled.
6. You can modify the value in a Watch window when the program is stopped or changing slowly. You can modify a variable in a Memory window anytime (see next section).

Add the SystemCoreClock Variable

1. Click on <Enter expression> twice and enter: SystemCoreClock
2. Right click on the Value and deselect Hexadecimal Display. 32 MHz will be displayed:
   

Memory Window

1. Right click on g_msTicks and select Add ‘g_msTicks ’ to… and select Memory 1.
2. Note the changing value of g_msTicks is displaying its address in Memory 1 as if it is a pointer. This is useful to see what address a pointer is pointing to. But this not what you want to see at this time.
3. Right click in Memory 1 and select Unsigned Long to see the data field as 32-bit numbers.
4. Add an ampersand & in front of the variable name g_msTicks and press Enter. Now the physical address is shown (0x2000_0008) in this case. This physical address could change with different compilation optimizations.
5. The data contents of g_msTicks is displayed as shown:
   
6. Right click on the memory data value and select Modify Memory at 0x3000000C. Enter a value and this will be pushed into g_msTicks. Since g_msTicks is updated often, you will only see the new value displayed for a very short time.
7. Stop the application.

How It Works

μVision uses Arm CoreSight technology to read or write memory locations without using CPU cycles. Most of the time, this is non-intrusive and does not impact the program execution timings. The Armv7-M/Armv8-M are Harvard architectures. This means they have separate instruction and data buses. While the CPU is fetching instructions at full speed, there is plenty of time for the CoreSight debug module to read or write to memory without stealing any CPU cycles. This can be slightly intrusive in the unlikely event the CPU and μVision reads or writes to the same memory location at exactly the same time. Then the CPU will be stalled for one clock cycle. In practice, this use of additional CPU cycles never happens.

System Viewer

The System Viewer provides the ability to view certain registers of peripherals and the CPU core. In many cases, these windows are updated in real-time while your program is running. They are available only in debug mode. Go to Peripherals - System Viewer to open the peripheral windows. Select CPU core registers by going to Peripherals - Core Peripherals.

SysTick Timer

The example application uses the Arm Cortex-M system tick timer.

1. Go to Peripherals - Core Peripherals and then select System Tick Timer S (SysTick).
2. The SysTick Timer window opens:
   
3. Run (F5) the application.
4. While the program is running, type 0x10000 in the SysTick -> LOAD register and click in another register or press Enter.
5. The program execution will speed up. This is the power of Arm CoreSight debugging.
6. Replace LOAD with 0x007CFF. A Stop and Reset will also accomplish this.
7. When you are done, Stop the application and close the SysTick Tick Timer (SysTick) window.

Watchpoints

Watchpoints can be thought of as conditional (access) breakpoints. Most Armv7-M and Armv8-M based processors have four data comparators. Since each watchpoint uses two comparators, you can configure two of them. The Logic Analyzer uses the same comparators in its operations. This means in μVision you must have two variables free in the Logic Analyzer to use watchpoints. μVision supports only one watchpoint.

Configure a Watchpoint

1. Go to Debug - Breakpoints (Ctrl-B) to open the Breakpoints window.
2. In the Expression box, enter g_msTicks==0x44.
3. Select both the Read and Write Access.
4. Click on Define and it will be accepted as shown here:
   
5. Click on Close.
6. Run (F5) the application.
7. When g_msTicks equals 0x44, the program will halt.
8. You will see g_msTicks displayed with a value of 0x44 in the Watch window:
   
9. Delete the watchpoint by selecting Kill All in the Breakpoints window or use Kill all Breakpoints in Current Target.
10. Select Close.
11. Leave Debug mode.

Use a Watchpoint in the Stack Space

If you put a RAM address as the expression with no value, the next read and/or write (as you selected) will cause the program to halt. This can be particularly useful in the Stack. Set an address at some limit and if the program reads or writes this address, the program stops.

1. In this example a watchpoint is created with address 0x2000_0008.
2. Running the program, the first read or write will stop the processor.
3. The Command window shows the setting of this Watchpoint and its execution including the approximate instruction location: