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Digital timer/counters are used throughout embedded designs to provide a series of time or count related events within the system with the minimum of processor and software over-head. Most embedded systems have a time component within them such as timing references for control sequences, to provide system ticks for operating systems and even the generation of waveforms for serial port baud rate generation and audible tones.
They are available in several different types but are essen-tially based around a simple structure as shown.
The central timing is derived from a clock input. This clock may be internal to the timer/counter or be external and thus connected via a separate pin. The clock may be divided using a simple divider which can provide limited division normally based on a power of two or through a pre-scalar which effectively scales down or divides the clock by the value that is written into the pre-scalar register. The divided clock is then passed to a counter which is normally configured in a count-down operation, i.e. it is loaded with a preset value which is clocked down towards zero. When a zero count is reached, this causes an event to occur such as an interrupt of an external line changing state. The final block is loosely described as an I/O control block but can be more sophis-ticated than that. It generates interrupts and can control the counter based on external signals which can gate the count-down and provide additional control. This expands the functionality that the timer can provide as will be explained later.
Timer/counters are normally defined in terms of the coun-ter size that they can provide. They typically come in 8, 16 and 24 bit variants. The bit size determines two fundamental properties:
• The pre-scalar value and hence the frequency of the slowest clock that can be generated from a given clock input.
• The counter size determines the maximum value of the counter-derived period and when used with an external clock, the maximum resolution or measurement of a time-based event.
These two properties often determine the suitability of a device for an application.
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