Save valuable picoseconds using ECL-wired OR

Often, when you are designing with high-speed ECL (emitter-coupled logic), you have too little time between clock cycles to implement logic functions using gates between flip‑flops. In these cases, you can derive equivalent-logic functions using the wired-OR and flip‑flop complementary inverted outputs (reference 1, reference 2, and reference 3). You can parallel the emitter-follower outputs of ECL with a pulldown resistor to implement the OR function with almost no time-delay penalty. Complementary outputs—one inverted—provide delay-free logic inversions.

This Design Idea uses the older Motorola 10H ECL logic family, the fastest available when I was building the design (Figure 1). Newer ECL families are much faster, but the same wired-OR principle applies. For clarity, the figure omits power and 50Ω pulldown resistors. This design needed an XOR comparison between a PRBS (pseudorandom-binary-sequence) data stream and a local PRBS reference for a BER (bit-error-rate) counter running at 250 Mbps (Figure 1a). A problem occurred with the design, however: The clock period at 250 Mbps is 4 nsec, whereas the 10H107 XOR/XNOR gate’s maximum propagation delay is 1.7 nsec. In addition, the 10H131 flip-flop’s maximum propagation delay is 1.8 nsec, and the required input-setup time is 0.7 nsec. All these delays total 4.2 nsec, which exceeds the 4-nsec clock period by 200 psec. Adding a fourth flip-flop with wired-OR outputs to replace the 10H107 XOR/XNOR solves the problem (Figure 1d).

The XNOR-equivalent function uses NOR, AND, and OR functions (Figure 1b). The circuit in Figure 1c separates the NOR into the equivalent OR with an output inverter and converts the AND into the equivalent OR with inverted inputs and output. Now, the circuit uses only ORs and inverters. This form is necessary for implementing the wired-OR equivalent (Figure 1d). In this case, the inverted-complementary outputs of the flip-flops replace the inverters, and a parallel electrical connection between the flip-flops’ outputs replaces the OR gates.