A 21st Century Sea Change Taking Place in Embedded Microprocessors (cont.)
Real Time Clocks
As traditional processors have grown in processing speed and complexity, they have moved further and further away from their ability to handle tasks in real time, meaning the time to process code is indeterminate and will vary from cycle to cycle. This is largely due to the introduction of increasingly larger caches used by the processor to reduce external memory accesses. Thus, on one loop through the code the instructions are all fetched externally, but on the next they are contained within the cache. At the same time, as processor complexity has grown, the number of CPU registers has increased as well. Accordingly, the amount of time required to save the contents of those registers during interrupt handling has increased. All of this makes modern processors ill-suited for embedded applications, to say nothing of the large memory requirements and sheer chip cost.
Embedded processors have always stressed the ability to handle real time applications, to process code in a guaranteed time slot, to handle events and displays within a tightly controlled (and shrinking) time allotment. Single processor chips use a real time clock, supplied by an external reference, to setup and control those tasks. But what is the ideal arrangement in a multicore chip?
Thinking about the application as a set of related tasks and subtasks, with cores assigned to each, provides an answer. Modern applications, especially those that are multimedia intensive, are not characterized by one or two tasks that must be accomplished within a time slot. Today, many if not most of the tasks have a real time component to them. Consequentially, one core will need to have access to the real time clock reference which it uses to inform the other cores by sending status signals to them in the form of messages, or each core must have the capability of accessing that reference clock directly. Of the two, the latter is a much better solution.
If status signals can be eliminated by each core having its own access to the real time clock, that combined with the lack of need for status signals to transfer data between cores, goes a long way to eliminating the status signal form of communication between cores altogether. Notice we are not suggesting that a system clock signal be distributed across the cores requiring millions of nodes to be switched synchronously to the beat of that clock. For the real time clock to be effective, only a handful of nodes in each core must be switched, and the effect on power dissipation is negligible. A simple counter on each node is more than sufficient to make each node self-sufficient in terms of real time processing.
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