Synchronization and Arbitration in Digital Systems

3

Circuits

In the design of a synchronizer the values of T_w and τ depend on the circuit configuration. Both affect the resolution time, but τ is more important for synchronization because the synchronization time needed is proportional to the resolution time constant τ. The effect of increasing T_w by a factor A is simply to add to the synchronization time an amount equal to τ ln(A).

T_w is mainly determined by the input characteristics of a latch circuit and τ is the time constant of the feedback loop. To some degree these two can be traded, a low-power input drive can reduce the loading on the feedback inverters thus reducing τ, but usually at the expense of T_w. Increasing power can often reduce both T_w and τ because the parasitic capacitances become a lower proportion of all capacitance, but only up to a point where parasitic capacitances become negligible and the value of C/G reaches a minimum.

3.1 LATCHES AND METASTABILITY FILTERS

Figure 3.1 shows a simple latch made up of four NAND gates and an inverter. When the clock goes low, both R1 and R2 go high and the latch becomes opaque. Without the two inverters on the outputs a metastable level of V_dd/2 could cause any following digital circuits to malfunction when the circuit becomes metastable. The inverters on the outputs of Figure 3.1 prevent the half-level appearing at the output because they have a lower than normal threshold level. If the latch is in a metastable state, both inverter outputs are low because the output level ...