The LPC13xx datasheet suggests some clock setup is necessary to use USB. Does the chip's built in boot loader make it possible to load the initial program over USB, or do the clock multipliers have to be configured with JTAG first?
Details of USB booting are provided in the User Manual on page 294:
The LPC134x is enumerated as a Mass Storage Class (MSC) device to a PC or another embedded system. In order to connect via the USB interface, the LPC134x must use the external crystal at a frequency of 12 MHz. The MSC device presents an easy integration with the PC’s Windows operating system. The LPC134x flash memory space is represented as a drive in the host file system. The entire available user flash is mapped to a file of the size of the LPC134x flash in the host’s folder with the default name ‘firmware.bin’. The ‘firmware.bin’ file can be deleted and a new file can be copied into the directory, thereby updating the user code in flash. Note that the filename of the new flash image file is not important. After a reset or a power cycle, the new file is visible in the host’s file system under it’s default name ‘firmware.bin.
Page 291 describes how to connect the device pins to ensure that USB booting takes place.