External crystal for USB needed?

[claudio_9171]

Posted on September 29, 2015 at 04:37

Is the STM32 series capable of handling low speed USB without external crystal?

Best regards.

[Tesla DeLorean]

Posted on September 29, 2015 at 05:41

Which STM32 specifically? Most of them need a crystal to get remotely close to the required accuracy/stability needed for USB.

Some of the newer offerings have a trimmed HSI/MSI capable of generating a suitable 48 MHz reference, you should review the check-box items in the assorted press-releases and product pages.

[claudio_9171]

Posted on September 29, 2015 at 07:53

Thank you for quick reply. Yes, now I see that ST does indeed explicitely feature some of the MCUs as working ''crystalless''. Good. Best regards

[claudio_9171]

Posted on September 29, 2015 at 08:07

One additional question. I'm planning to use it for a keyboard controller. Since STM32 MCUs come with bootloader, I don't really need a programmer, to get my firmware on it, right?

[Tesla DeLorean]

Posted on September 29, 2015 at 08:11

Most have a USB DFU programming scheme, though if you want any chance of developing and debugging on the platform you really need some form of SWD/JTAG pod.

[claudio_9171]

Posted on September 29, 2015 at 08:26

I see. So it's better to include a SWD socket into the design, for whatnot may happen. Good. Thanks you, clive1

[Tesla DeLorean]

Posted on September 29, 2015 at 08:40

You can do it as test points, edge castellations, or whatever floats your boat. Systems without this connectivity in some form are real hard to trouble-shoot. I'd recommend SWDIO, SWCLK, SWO, NRST, GND as a minimum, and ideally also USART1_TX and USART1_RX (or equivalent as called out in the System Loader (ROM) specs)

If you have space, using standardized connections will also save to lot of time and messing around.