chipKIT® Development Platform

Inspired by Arduino™

Operate without a crystal as a USB device?

Created Mon, 28 Sep 2015 15:13:04 +0000 by watchdogtimer


Mon, 28 Sep 2015 15:13:04 +0000

Some of the 8-bit PICs (e.g., 18f2Xk50) can operate as a full-speed USB device without a crystal or resonator by self-tuning it's clock to the USB clock of a host computer.

Is this possible to do with the 32MX250F128B? The data sheet seems to suggest that it's possible (the master clock can be tuned run-time with the USB clock), but I haven't been able to find any examples or any explanation of how to do this.

If it's possible, is there a bootloader available that would be compatible with doing this?


Mon, 28 Sep 2015 20:59:43 +0000

No, it's not possible to run USB from the internal FRC.

The FRC is used for USB but not for running it - it's only used to bring it out of suspend mode, after which it must switch to the crystal oscillator.

6.3.4 USB Clock (USBCLK) GenerationThe USBCLK can be derived from the 8 MHz internal FRC Oscillator, 48 MHz POSC, or the 96 MHz PLL from the POSC. For normal operation, the USB module requires exact 48 MHz clock. When using 96 MHz PLL, the output is internally divided to obtain 48 MHz clock. The FRC clock source is used to detect USB activity and bring USB module out of Suspend mode. Once USB module is out of Suspend mode, it must use a 48 MHz clock to perform the USB transactions. The internal FRC Oscillator is not used for normal USB module operation


Tue, 29 Sep 2015 02:52:13 +0000

I do so hope that Microchip will be migrating this feature into the PIC32 parts at some point in the future. It seems really cool.