Many LPC55 users experienced connection failure when using ISP USB0 for firmware update. In practice, we don’t suggest user updating firmware via ISP USB0 for LPC55(S)6x/ 2x,LPC55(S)1x/0x parts.
LPC55 USB0 is Full Speed USB port. The default setting of CMPA turns off the USB0 port. Some users may reconfigure CMPA to enable ISP USB0 in order to use ISP USB0 BOOT, but this is not recommended in practice.
LPC55 ISP USB0 uses internal FRO as clock source. According to LPC55 data sheet, the FRO accuracy is only +-2%, while the FS USB data rate tolerance specification is +-2500ppm(+-0.25%). Obviously, the LPC55 FRO spec can’t meet the USB0 clock accuracy requirement. See below extraction from NXP manuals.
Fig 1. The accuracy of FRO ( Extracted from LPC55S69 Datasheet )
Fig 2. The accuracy requirement of USB FS( Extracted from TN00063 )
Some users may wonder why USB0 can use internal FRO as clock source in the user application? Whenever internal clock source FRO is used as USB0 clock source, we must calibrate FRO in source code for communication. That’s to say, trim FRO to an accurate frequency. We can see FRO trim in many MCUXPressoSDK USB demos. When using FRO as the USB0 clock source, in order to ensure the USB0 clock accuracy, we must use the USB0 SOF frame synchronization to calibrate the FRO in order to ensure the accuracy of FS USB clock source (reference design of TN00063, TN00063-LPC5500 Crystal-less USB Solution). Unfortunately, the BOOT ROM of LPC55 does not support USB SOF calibrating FRO. As a result, even if we enable ISP USB0, the FRO clock drift can still cause USB0 communication failure under non-room temperature conditions.
Since ISP USB0 is not recommended for firmware update, the user manual no longer announces the enablement bit of ISP USB0 in CMPA. If you need to use USB0 for firmware update, we recommend using ISP USB1 (High Speed USB), because USB1 uses accurate external clock source which can ensure the ISP USB1 working stable.