Function of DISC circuit, USBP and BOOT0

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Function of DISC circuit, USBP and BOOT0

(17 posts) (5 voices)

Started 4 years ago by nplayle
Latest reply from siy

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nplayle
Member

Hi all,

Looking at the Maple rev5 schematic I have a few questions.

First, what is the function / purpose of the DISC circuitp It seems to be some sort of amplifier but for what purpose? Can it be omitted?

Second, BOOT0 is tied to PC9, could it be moved to say, PB0 or PB1? It is tying up a remapped UART I'd like to be able to use if its possible.
Same with USB_P, it's tying up UART3TX on PC11, could it be moved to a different pin as well with some modifications to the bootloader? I assume its purpose is to detect the presence of a USB cable?

Thanks,
Nick

Posted 4 years ago #
siy
Member

The purpose of DISC circuit is software simulation of USB disconnect. This is necessary to switch between USB modes (initial mode is DFU then board is switched to ACM) without physical disconnect. The circuit can be changed, but if you need to have board properly re-enumerated after mode switching you still need this circuit. Pin which controls this circuit can be changed (with appropriate changes in the bootloader). If you're going to go this way and change bootloader, it is worth to use maple-RET6 branch of maple-bootloader git repository. It is a bit better polished, in particular all code related to pin assignment is refactored and pin can be changed by simple changes in config.h file. With master branch code such a change is much more painful. Button pin can also be changed there and bootloader will work, but you'll need also make changes in libmaple (take a look at libmaple/wirish/boards).

Posted 4 years ago #
siy
Member

By the way: the DISC circuit can be simplified in several ways for most uses. Original circuit follows USB specs precisely and does not perform disconnect if there is no VBUS voltage. This is not necessary in wast majority of cases. With some change in the logic of bootloader it can be reduced to single resistor (original circuit is inverting). I've took slightly different approach, my boards using circuit which can be seen on this picture: http://we.easyelectronics.ru/uploads/images/00/22/11/2011/12/27/d14b13.png. Recent versions of my boards use different component for ESD protection, but the idea of circuit remains the same.

Posted 4 years ago #
mikep
Member

This is interesting.
How big are the two resistors on the base of the PNP transistor in your circuit?

Posted 4 years ago #
siy
Member

There are no resistors at all (I mean as separate components). Resistors are part of symbol of "digital transistor" which has these resistors built in.

Posted 4 years ago #
siy
Member

Sorry, didn't understand that you mean resistance. Component referred at circuit has 10K resistors built in, if I remember correctly. But I think there is no difference and any PNP digital transistor will work here.

Posted 4 years ago #
mikep
Member

Yes, that's what I meant, the resistance value. So it's 10k each. Thanks!

Posted 4 years ago #
nplayle
Member

I like the looks of that circuit mikep, specifically that it gets rid of all the external components. My board is pretty cramped and that is the main reason for wanting the change.

Regarding the RET6 branch, does it require much work to get it working with the RBT6? I only ask because I have a bunch of sample RBT6 chips lying around I'd like to put to good use, and the extra features on the RET6 aren't of great importance to me.

Regarding changing the button and DISC port, I assume it would just require changing the following lines:

/* USB configuration. BOARD_USB_DISC_DEV is the GPIO port containing
* the USB_DISC pin, and BOARD_USB_DISC_BIT is that pin's bit. */
#define BOARD_USB_DISC_DEV GPIOC
#define BOARD_USB_DISC_BIT 12
#define BOARD_BUTTON_PIN 38

Posted 4 years ago #
siy
Member

RET6 and Maple r5 have identical schematic, the only change is different chip version used as MCU. To build bootloader for RBT6 from RET6 sources you need to do following changes:
1. Set FLASH_PAGE_SIZE to 0x400 in config.h
2. Replace stm32_lib/c_only_md.ld with one from master branch (or edit it manually, set LENGTH for RAM 20K and for FLASH 128K)

I've just checked this approach with one of my boards and so far built bootloader works fine.

Posted 4 years ago #
siy
Member

In regard to changes required for different DISC pin: as far as I understand, things mentioned above should be enough to make bootloader work properly. Unfortunately I have no hardware to check this in details (all my boards share DISC and BUTTON pin assignments with Maple r5).

Posted 4 years ago #
ala42
Member

You also have to change the USB_DISC_DEV and USB_DISC_PIN defines in usb_config.h. I simply use a 470 Ohm resistor to pull down the D+ line, but this requires inverted software logic and an open drain output mode settings.

Posted 4 years ago #
Rod
Member

The Olimexino board does something similar in using a DTA114YKAT, but it pulls USBDP+ low (as does Maple) instead of VBUS.

sly I wonder if you have a comment?

Posted 4 years ago #
siy
Member

Well, my circuit also does pull USBP. If you look into datasheet for STF202 (for example, here: http://www.onsemi.com/pub_link/Collateral/STF202-22T1-D.PDF), then you'll notice that VBUS pin has only two internal connections - ESD protection diode and pull up resistor. My circuit just uses this resistor instead of external component. In fact any upstream USB protection IC has similar pull up resistor with same value (1.5K) as it is necessary to satisfy USB specs. In my recent boards I'm using USBUF02W6 for this purpose and I think that STF203 would work fine too (had no chance to purchase them in local stores).

Posted 4 years ago #
nplayle
Member

OK I think this is starting to make sense now, thanks guys.
Looking at the Olimexino board (http://www.olimex.com/dev/DUINO/OLIMEXINO-STM32/OLIMEXINO-STM32_Rev.A-schematic.pdf ), they use a pull up resistor on the D+ line beacuase the transistor they use doesn't contain a pull up resistor, whereas your (siy) circuit contains a PU in the STF202?

Changing the DISC pin is possible via changing #defines in maple.h and usb_config.h

Use the RET6 branch of code because it's nice

Anyone have any input on the possibility of switching the button pin from PC9 to a different pin? I assume it would be possible since it looks like BOOT0 is just being taken off as a signal for the bootloader?

Posted 4 years ago #
siy
Member

Yes, my circuit contains PU in STF202 (or in USBUF02W6 which is basically identical to STF202 from this point of view).

Changing button pin is definitely possible because different versions of Maple have it connected to different pin - PB8 for Mini, PC9 for Maple/Maple RET6, PG15 for Native (in fact this is the same pin as BOOT0). Beside bootloader this affects libmaple (BOARD_BUTTON_PIN definition, at least). Perhaps it might be helpful in your case just compare board definitions in libmaple for Maple and Olimex STM32-H103. Since boards are identical by the means of the MCU and basic hardware but have different pin assignments, all relevant differences in code will be clearly visible. As far as I know there is no bootloader for Olimex board (at least I saw nothing like that).

Also, if you're interested, I can send you my version of board.

Posted 4 years ago #

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