Title: STM32 Serial Wire Viewer & Output, ST-LINK: Beginner Questions on SWO, SWCLK & Reset

Question

Hi I turned around pretty hard. I am asking for your understanding because I am just starting with STM32, or at least I have been trying for some time. I'm not jumping into deep water, I'm just interested in the topic. I realize that these are obvious things to you, but I don't have...

Sareph · Accepted Answer

It's kind of a one-way serial port to put whatever information you see fit on it. Of course. Yes. Like all CPUs, it will do what you ask it to do, so if you enable trace and give it data, it will send whatever you want there. NO. If you turn off the SWD in the program, this connection during the reset gives you access to the SWD interface. And yes, you can hold the reset, but if you don't have a Chinese clone, the programmer will do it for you. Probably because SWD has two signals.

BlueDraco · Answer

On the contrary. This option means that ST-Link will try to reset the CPU by whatever method it can when attempting to connect. In some uCs the connection fails without it (e.g. often in the F4 series). If you programmed the SWD by mistake and you don't have the RESET line connected to the ST-Link, you can press the RESET button while connecting and it will work without this option, because in this case you reset the uC, not the ST-Link.

vp32 · Answer

I read a little, there is light at the end of the tunnel.
Isn't it a coincidence that after a hundred pins for SWD work are set in CubeMX so that the Debugger can be used during normal operation, otherwise they are ordinary GIPO? And maybe that's why it's connect under reset?
I also read that SWO also needs SWD. And now I don't understand SWD is two-pin and what about SWO, does it not connect together and actually three lines are needed?
Also tell me if SWO is activated in the ARM program or is it only activated in the program like Atolic? Because since it is a regular PB3 port, it can be used for other purposes.
P.S. why there is no ST-LINK programmer selection in Sytem Workbench. No underlined tab?

PS> now I've read that SWO is independent of SWD and can also work with JTAG, so what activates SDO?

Sareph · Answer

SWD as such is two-pin, SWD + async trace is three-pin. So if you want to use ITM then you need 3 lines. You can initialize the ITM in the program code on the MCU, the debugger can do it for you. If you're not using it as an SWO, then yes - it can be used as a GPIO. And basically you can use trace/ITM even without SWD/JTAG but then *probably* only in synchronous mode.

vp32 · Answer

Can you help me?
I don't understand why I can't connect to uC using ST-LINK.
SWD pins PA13 and PA14 are free, I don't have them occupied. In Cube they are set as SWDIO and SWCLK. Supposedly, this setting does nothing other than not assigning pins to other purposes.
In TrueStudio, there is always an error after trying to establish communication
Error in initializing ST-LINK device.

When I run ST-Link Utility, it does not connect either, but after pressing RESET, it manages to establish communication with the board.

What's the deal with Connect Under Reset, why can't I connect via Debbuger in TrueStudio?

Sareph · Answer

Well, it depends, in F1, for example, the default setting is full JTAG. In F0, SWD is enabled by default, but... ... it would turn out that you turned off the SWD, most likely in GPIO by changing the pin functions in port A.

vp32 · Answer

Ok, how to fix it?, The chip is STM32F103C8Tx
So it comes out that JTAG is the default, but I read that if there is JTAG, SWD switches when trying to establish a connection. or not?

Tell me what is this Connect Under Reset, why did they separate it specifically?

Sareph · Answer

A, then not in port A, because there DP is configured in AFIO->MAPR I haven't actually checked, somehow I always have the SWD manually selected. That is why. Because you might want to connect a debugger while the program is running too, like if it crashes, to see what's going on. Or maybe there is a situation like you have, that for some reason the SWD is turned off (or it will stop working because you set the clocks wrong or you do not connect AVCC) and somehow you have to get to the system. Check if: AFIO->MAPR = (0b010

vp32 · Answer

No changes unfortunately I don't understand what you mean, if not on A then on what. I took PA13 and PA14 from CubeMX. See drawing. Manually, i.e. by typing in that instruction you posted? And if Under Reset is what special initialization is it? Couldn't you write in the note that if you don't want to connect, then cut RESET? I can't figure out the intention of this special mode.

Sareph · Answer

Your Cube configuration indicates that you have SWD *disabled*. Go under device SYS, there enable debug on SWD or async trace if you want SWO. Merely marking the pins here does not give you anything. (Orange - pony reserved but device is not turned on). On A, but no. Because in older STMs, if you want to switch SWD configurations, you change it in AFIO, and if debug is turned on, these pins are force intended for debug and you can change their functions in port A (GPIO) and it won't help you anyway. Yes. At the same time, it will also switch all remapped pins to default ones. Well, that's just for the test. Reset presses the programmer for you, unless you have a Chinese clone, then no, because they do not have a hardware reset and you have to do it manually.

vp32 · Answer

You got me so confused I have no idea what you're talking about Correct me if I misunderstand you. I connect the programmer to these Pins PA13 and 14. Bits 26:24 SWJ_CFG[2]: Serial wire JTAG configuration These bits are write-only (when read, the value is undefined). They are used to configure the SWJ and trace alternate function I/Os. The SWJ (Serial Wire JTAG) supports JTAG or SWD access to the Cortex debug port. The default state after reset is SWJ ON without trace. This allows JTAG or SW mode to be enabled by sending a specific sequence on the JTMS / JTCK pin. 000: Full SWJ (JTAG-DP + SW-DP): Reset State 001: Full SWJ (JTAG-DP + SW-DP) but without NJTRST 010: JTAG-DP Disabled and SW-DP Enabled 100: JTAG-DP Disabled and SW-DP Disabled Other combinations: no effect If the value 000, 001, or 010 is written there (in these bits 24:26), these PA13 and PA14 will always be on JATG SWD. And it doesn't matter what he writes to the GPIO port because it doesn't matter then. So for these PA13 and 14 to do anything, those 24:26 bits must be 100? Yes? You write in older and newer, how and which, how do I know which is old and which is new? But reset isn't meant to be pressed all the time? because if I hold it all the time it doesn't work? or so? Now try with SYS, pony turned green Added after 7 [minutes]: But this setting in SYS SWD gives the same effect as your instruction because according to CubeMX it gives:

Sareph · Answer

Well, when JTAG is on, it's not only them, but roughly yes. Yes. From the documentation, the newer ones don't have AFIO. And they have other GPIO registers. If you have properly configured and connected SWD, then you shouldn't have to hold the reset (unless something with the program goes very wrong). Because it's roughly a different way to accomplish the same thing (if you prefer to use Cube). Well, my instruction changes the pin remapping settings of other devices, if they are set, Cube does not.

vp32 · Answer

Okay, something happened after changing SYS to SW Debug.
But that gave me another question. Because setting this SW Debug causes the function to be called

Log in, to see the code

Which registers AFIO->MAPR 0b010 at [26:24]
If I don't set SW Debug it will be entered into these registers

Log in, to see the code

So they will be ordinary GPIO

But why, after all the initialization done by CubeIDE, I type it manually

[syntax=c]AFIO->MAPR =(0b010

vp32 · Answer

P.S. thanks for the help

excray · Answer

And what does your ST-Link look like? Sareph already mentioned that Connect under reset doesn't work in Chinese versions.

vp32 · Answer

It's a typical quinol.
I rather meant the principle of operation of this Connect Under Reset and not whether my programmer works. are these instructions sent when the connection is established, or is it just a matter of giving an additional reset signal and if so, for how long?

Sareph · Answer

You download the reset to ground, the programmer initiates what it needs in the CPU during this reset, you release the reset (or the programmer does it) and the SWD is restored, so you can program.

Title: STM32 Serial Wire Viewer & Output, ST-LINK: Beginner Questions on SWO, SWCLK & Reset

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