Content originally posted in LPCWare by lpcmunich on Thu Oct 29 06:02:57 MST 2015
Hi rjm,
Please find my comments in-lined below.
The user manual UM10736 states in Table 91, that ISP_1 defaults to PIO1_11. That's clear from other parts of the documentation. However, USB_VBUS is declared as movable function, and the associated register defaults to 0xFF. But this should mean that USB_VBUS is not assigned by default to a pin at all. So, the init sequence should be responsible for this. Is that correct?
>> Yes, the Boot code (in ROM) takes care of necessary initialization.
Another question: Why is it stated that VBUS should be assigned to the ISP_1 pin for ISP usability over USB?
>> Please refer the Table-89 in UM10736. The ISP_1 is used to detect the USB cable connectivity (through VBUS).
Normally the ISP_1 is low (no USB cable connected). But when the USB cable is connected, the ISP_1 goes high since the VBUS signal is high.
A sample schematic (the LPCXpresso eval kit with LPC1549) shows that no Pull-Up is present on ISP_1. When I would not use a USB cable, the MCU would reset in an ISP-Boot-mode.
>> As explained above, the part enters the ISP mode via USB when ISP_1 is high and not Low.
Hence I'm tempted to leave ISP_1 without any connection except a pull-up resistor (as is the case with ISP_0 in my own board, and then assign VBUS to another pin.
>> If you would like to use ISP via USB, you shouldn't connect a pull-up on ISP_1.