I am troubleshooting a commercial consumer USB HID which uses the CY8C24894-24LTXI .
I do not have a MiniProg to debug it or program it.
The board developed an issue, the cause of which is unknown to me, where it fails if initially powered with a voltage above approx. 3.45V. Simply plugged into a computer USB port, the device fails to be recognised/enumerated by Windows (device descriptor failed).
The board is USB powered via bus (not self powered via external power supply). There are no indications that it's meant to work at 3.3V (no 3.3v LDO regulator on board that I can see, and the LEDs on the physical device itself are too dim on 3.3V). There is no physical damage visible on the board, bypass caps are present (capacitance or ESR was not measured, all ceramic caps no electrolytic), although the inline resistors for the D+/D- line are 27Ohm, which is outside the recommended values as per the datasheet. Regardless, the device was working without incident for several years and still works at 3.3v which implies that there is no catastrophic/dead shorts on board.
Using a bench power supply to investigate the issue, I have found the following.
If initial voltage below 3.45V is applied, the device works fine. If Voltage greater than that is applied on startup, the device hangs and Windows does not recognise it (error 43 type, device descriptor failed).
However, if the board is booted with <3.45V (ideally 3.3v) then the voltage can be manually increased until 3.95V (this value is consistent), upon which the chip hangs and USB traffic ceases.
The internal 3.3v regulator seems to work, at least superficially, as I can detect some 3.25v traffic on the USB data lines when trying to boot it at 5v.
When booted at 3.3v and then manually raised up to 3.95v the voltage of the traffic on the usb lines seems to rise to around 3.45v.
I tried to see if the Voltage on the USB data lines goes out of spec as per the datasheet (+3.6v or -0.3v) but I got not triggers on the scope at those levels.
I have thoroughly checked the documentation and app notes but I did not find anything that fits the failure conditions/triggers.
Your help and guidance would be greatly appreciated.