It is a widely used design practice to designing intellectual property (IP) electronic modules with multiple asynchronous clock domains. This practice permits designers to meet performance requirements of each clock domain independently. This also eases the timing closure problem because each clock domain can be treated independently for optimization, clock tree synthesis and timing closure.
Providing multiple asynchronous clock domains enables clock-gating each domain independently to save dynamic power. In peripheral IPs the core clock can be clock gated when there is no data to be exchanged with the external world. The input/output (IO) clock can be kept on to generate a core/CPU interrupt on detection of incoming packet data from the external world.
With shrinking process geometries, static/leakage power has become a major contributor to total power consumption. Such static power can be kept low by operating at a lower supply voltage. Since lowering the voltage reduces how fast the circuit can operate, it is important to operate the device at a supply voltage that is just enough to meet the performance requirements. This voltage gives the best power solution without compromising performance.
The asynchronous boundary inside the peripheral IP also provides an opportunity to save on leakage power by creating separate voltage domains for each of the asynchronous clock domains. Existing electronic design automation (EDA) tools require that each asynchronous clock domain must be enclosed by a distinct logical hierarchy, which subsequently becomes a voltage island. Each voltage island can then be separately optimized in operating voltage and frequency to meet the power and performance goals.
In many systems some of these voltage islands may be un-used for long periods of time. These voltage islands can be independently powered off using power switches on the chip to save on both leakage and dynamic power.
Designs with multiple asynchronous clock domains can be exploited to:
1. Meet performance requirements of each domain independently;
2. Save dynamic power by clock-gating each domain independently;
3. Save static power by supplying each domain in a voltage island an optimum supply voltage; and
4. Save power by shutting off power to a voltage island when not used.
Legacy IPs are typically designed to meet only the first design objective. Thus they may not have these asynchronous clock boundaries along logical hierarchies. To meet the other objectives, the circuit should be partitioned along the asynchronous clock domain boundaries and logical hierarchies should be created which then can be mapped to voltage islands.