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How to Run Simple Code on the Cortex M4

 
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This article explains how to run code on the Cortex M4 core.

Walk-Through

Let's first start with the actual work. Use the Reg Access Tool to access the following physical memory locations.

# Address Value Operation Comment
1 0x3f04'0000 0x3001'a101 Write
2 0x3f04'0004 0xe7fc'6008 Write
3 0x4006'e028 0x3f04'0001 Write
4 0x4006'b08c 0x0001'5a5a Write
5 0x3f04'0008 Read Repeat this step. You will see incrementing values.

That's it. You are already done.

Explanations

Let's look at the details:

M4 Application

This is the assembly code that we want to run on the Cortex M4:

      0x3f040000      a101    ADD     R1, PC, #4     ; load counter address [0x3f040008] into R1
  .-> 0x3f040002      3001    ADD     R0, 1          ; increment counter
  |   0x3f040004      6008    STR     R0, [R1, 0]    ; store new counter value
  `=< 0x3f040006      e7fc    B       0x3f040002


Code and variable are located in the SRAM. The memory map looks like this:

Address Range Content
0x3f04'0000 - 0x3f04'0007 Program code
0x3f04'0008 - 0x3f04'000b 32bit counter

Steps to Run the Code

  • In steps #1 to #2 we write the assembly code to the SRAM.
  • In step #3, we write the M4 starting address into the SRC_GPR2 register. Bit 0 is set to start in THUMB mode.
  • In step #4, we start the M4 by setting the AUX_CORE_WAKEUP bit in the CCM_CCOWR register. Reading back this register will show 0x00010000.
  • Step #5 is only to verify that the application is running.