The invention provides a testing circuit for testing a connection between a chip and external circuitry. A current source is used to inject a DC current towards the connection to be tested from the chip side. On-chip ESD protection is provided giving a path between the connection to be tested and a fixed voltage line. A shunt path is also coupled to the connection to be tested on the external circuitry side. It is determined if the current source current flows through the ESD protection circuit, and this can be used to determine whether or not the connection to be tested presents an open circuit for the DC test current.