Connections that will stop the enumeration process are as follows:
Power missing. If any of the AVCC and all VCC pins are not powered with 3.0 to 3.6 volts the part may not start.
EA connected external memory without programs loaded into the external memory. Connecting EA to ground will link to internal memory. If an EEPROM is used with EA high you cannot do a download from EEPROM to external memory and must have external EPROM (or other non-volatile memory) programmed. It is best to start with EA at ground.
|No pull-ups on SCL and SDA. The way the chip works is it will look for an EEPROM on the SCL, SDA lines. To do this they look for inactivity on these pins and if no activity the chip will take control of the pins and look for an EEPROM at address 0 or address 1. If no EEPROM is present then it will start with defaults. Therefore you must have pull ups to show no activity (high is idle state).|
|4)||Proper crystal. The start up load capacitance is wide, normally you have between 20 and 33 pF with a matched crystal. We use 22 pF with a crystal made for 22 pF, designs have powered up with 33 pF matched to 33 pF load capacitance crystal. The drive level of the crystal must be at least 0.50 mW.|
|5)||Ground the appropriate reserve pins (follow the data sheet). There are test mode pins if not connected per the data sheet some will be floating or high and the design may be in a test mode and not communicate.|
|6)||If the chip has no EEPROM then it will not depend on address/data lines, all port I/O will be inputs and not matter.|
|7)||If wakeup starts in high, the part is allowed suspend for default conditions, low inhibits it. The capacitor and resistor used in the dev kit allows the FX2 to inhibit wakeup at start up and allows the clock to show on the scope.|