The 32-bit counter counts up or down synchronously with the Source signal. The Gate signal and other counter inputs are asynchronous to the Source signal, so the USB-6421 synchronizes these signals before presenting them to the internal counter.

100 MHz Source Mode

In 100 MHz source mode, the USB-6421 synchronizes signals on the rising edge of the source, and counts on the third rising edge of the source. Edges are pipelined so no counts are lost, as shown in the following figure.

Figure 101. 100 MHz Source Mode


External Source Greater than 25 MHz

With an external source greater than 25 MHz, the USB-6421 synchronizes signals on the rising edge of the source, and counts on the third rising edge of the source. Edges are pipelined so no counts are lost, as shown in the following figure.

Figure 102. External Source Greater than 25 MHz


External or Internal Source Less than 25 MHz

With an external or internal source less than 25 MHz, the USB-6421 generates a delayed Source signal by delaying the Source signal by several nanoseconds. The USB-6421 synchronizes signals on the rising edge of the delayed Source signal, and counts on the following rising edge of the source, as shown in the following figure.

Figure 103. External or Internal Source Less than 25 MHz