A source synchronous external memory device returns data to a memory controller of an electronic device with its own clock signal, which allows the returned data to be captured with a high degree of accuracy. The captured data must then be resynchronized with the clock signal which controls the other components of the electronic device. This problem is particularly acute in the case of high speed memory devices, such as DDR (Double Data Rate) memory devices, and similar memory devices. Test data is written to the external memory device, and is then read back several times. Attempts are made to resynchronize the captured data with the system clock, with different relative variations in the phase of the captured data and the system clock, in order to select a value for the relative phase, which allows for optimum resynchronization.