A method of timing data sampling includes, in a data acquisition device, generating sampling intervals from a system clock of the data acquisition device, sampling data at the generated sampling intervals, and receiving start of frame (SOF) signals from a port, such as a USB port. For a selected number of SOF signals received, an actual number of system clock cycles is determined for a time interval corresponding to the selected number of SOF signals. The actual number of system clock cycles is compared to a nominal number of system clock cycles and a sampling interval is adjusted based on the comparison. The actual number of system clock cycles may be determined using a count-and-capture counter. The nominal number of system clock cycles may be calculated based on a nominal system clock rate and on a known SOF interval. Generating sampling intervals may include using a divide-by-N counter.