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<title>1.3 Flash – Boot – Debug</title>
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<h1>1.3 Flash – Boot – Debug</h1>
Now that I have an executable bootstrap, I need to flash an actual board
with it to check if it works as expected. On a member of the STM32F030
family, there are two options for flashing:
<ul>
<li> Use the Software Debug (<b>SWD</b>) interface with a ST-Link adapter and an
utility software to flash.
<li> Use the serial interface to communicate with the boot loader in System
Memory via an USB to serial adapter.
</ul>
As the bootstrap code I want to test does nothing except giving control
to an idle loop, I will need extra debugging on top of the flashing
functionality, so the SWD interface is a must for what I want to achieve
here.
<h2>ST-Link<h2>
<h3>1. SWD interface</h3>
Arm Serial Wire Debug Port (<b>SW-DP</b>) is provided as a two wire interface.
On the STM32F030, the functionality is activated at reset on two pins
(PA13=SWCLK, PA14=SWDIO). Most boards available online have pre-soldered
pins for Vcc=3.3V, Gnd, SWCLK and SWDIO.
<h3>2. ST-Link v2</h3>
ST-Link is an in-circuit debugger/programmer for the STM8 and STM32
chipsets. There are three versions of the product as well as mini
versions. STM32 Discovery and Nucleo boards have an onboard ST-Link v2.
I am using ST-Link v2 mini clones. For simple use cases, the ST-Link can
provide power to the board to flash or test.
<p>
<img src="img/13_stlink.png"
alt="ST-Link v2 mini clone connected to STM32F030F4P6 based board">
<h3>3. STM32 Cube Programmer</h3>
Referenced as
<a href="https://www.st.com/content/st_com/en/products/development-tools/software-development-tools/stm32-software-development-tools/stm32-programmers/stm32cubeprog.html">
STM32CubeProg</a>
on STMicroelectronics website, the STM32 Cube Programmer comes with USB
drivers and a firmware upgrade utility for the ST-Link. It’s a java
based application with available distribution for Win32, Win64, Mac and Linux.
There are regular updates to support the latest chipsets. I am currently
using version v2.17.0.
<h2>Roadtesting the Bootstrap</h2>
First I activate the connection in the programmer.
<pre>
14:37:31 : STM32CubeProgrammer API v2.17.0 | Windows-64Bits
14:37:38 : UR connection mode is defined with the HWrst reset mode
14:37:38 : ST-LINK SN : 55FF6B065177495619420887
14:37:38 : ST-LINK FW : V2J45S7
14:37:38 : Board : --
14:37:38 : Voltage : 3.27V
14:37:38 : SWD freq : 4000 KHz
14:37:38 : Connect mode: Hot Plug
14:37:38 : Reset mode : Software reset
14:37:38 : Device ID : 0x444
14:37:38 : Revision ID : Rev 1.0
14:37:38 : Debug in Low Power mode is not supported for this device.
14:37:39 : UPLOADING OPTION BYTES DATA ...
14:37:39 : Bank : 0x00
14:37:39 : Address : 0x1ffff800
14:37:39 : Size : 16 Bytes
14:37:39 : UPLOADING ...
14:37:39 : Size : 1024 Bytes
14:37:39 : Address : 0x8000000
14:37:39 : Read progress:
14:37:39 : Data read successfully
14:37:39 : Time elapsed during the read operation is: 00:00:00.007
</pre>
Then program and verify the bootstrap code. Either binary, Intel hex or
Motorola S rec format are supported. Our <b>Makefile</b> has rules for binary
and Intel hex, <b><i>objcopy</i></b> also support Motorola S record as an output
format. Last build produced <b>boot.hex</b>.
<pre>
14:40:24 : Memory Programming ...
14:40:24 : Opening and parsing file: boot.hex
14:40:24 : File : boot.hex
14:40:24 : Size : 10.00 B
14:40:24 : Address : 0x08000000
14:40:24 : Erasing memory corresponding to segment 0:
14:40:24 : Erasing internal memory sector 0
14:40:24 : Download in Progress:
14:40:24 : File download complete
14:40:24 : Time elapsed during download operation: 00:00:00.130
14:40:24 : Verifying ...
14:40:24 : Read progress:
14:40:24 : Download verified successfully
14:40:24 : RUNNING Program ...
14:40:24 : Address: : 0x08000000
14:40:24 : Application is running, Please Hold on...
14:40:24 : Start operation achieved successfully
</pre>
Finally check the registers in the MCU Core Panel:
<pre>
MSP: 0x20001000
PC: 0x8000008
</pre>
After reset, the stack pointer has been initialized and the program
counter is on the idle loop under execution.
<p>
If I check the Programming Manual
<a href="https://www.st.com/content/st_com/en/search.html#q=PM0215-t=resources-page=1">
PM0215 <i>STM32F0 series Cortex-M0 programming manual</i></a>, I can
read the following about the registers <b>MSP</b> and <b>PC</b>:
<pre>
Stack pointer (SP) register R13
In Thread mode, bit[1] of the CONTROL register indicates the stack
pointer to use:
● 0: Main Stack Pointer (MSP)(reset value). On reset, the processor
loads the MSP with the value from address 0x00000000.
● 1: Process Stack Pointer (PSP).
Program counter (PC) register R15
Contains the current program address. On reset, the processor loads the PC
with the value of the reset vector, which is at address 0x00000004.
Bit[0] of the value is loaded into the EPSR T-bit at reset and must be 1.
</pre>
- According to this, initial values for <b>MSP</b> and <b>PC</b> registers are
fetched from address <b>0x00000000</b> and <b>0x00000004</b> respectively, but
I have located the isr table at the beginning of the Flash memory at
address <b>0x08000000</b>! This works because the memory space at address 0
is a mirror of another memory area. Which area is mirrored depends of
the state of the <b>BOOT0</b> pin. On the board I am testing, there is a
jumper to select either Flash or System memory by setting the state of
the <b>BOOT0</b> pin to high or low.
<p>
- The <b>ESPR T-bit</b>, mentioned in the description of the <b>PC</b>
register is the Thumb bit. As I highlighted before when I checked the
output of our first build, bit 0 of the second entry in our isr table is
set to 1 as requested by this specification.
<h2>Checkpoint</h2>
I used the Serial Wire Debug (SWD) interface to flash and debug our bootstrap
in an actual board using a ST-Link hardware adapter and STM32 Cube Programmer
application.
<p>
<a href="14_ledon.html">Next</a>, I will provide feedback of execution directly
through the board.
<hr>© 2020-2025 Renaud Fivet
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