Suggested improvements for micros() and the timer0 overflow ISR #48
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1) The 32-bit timer0_overflow_count has been renamed to timer0_micros and now counts in microseconds directly. The rename should be okay as none of the rest of the core refers to it. Since it was not static, it is possible that clients are referring to it in which case the old name s/b kept. 2) micros() no longer needs a 32-bit multiply to compute the result. Since the 32-bit counter is now directly in microseconds, all that is required is a 16-bit multiply of TCNT0 to add to the counter. This saves well over 20 instructions during execution, but he overall code is a few bytes larger. 3) micros() keeps interrupts disable for a shorter length of time now, resulting in slightly improved overall interrupt latency for other interrupts.
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Regarding (1) above, I realized that if clients (sketches) are referring to timer0_overflow, it doesn't really matter if that's renamed or not -- changing the units from overflow count to microseconds would break them anyway. The validity of this whole idea depends on whether clients are using timer0_overflow_count or not (or if they are, is it okay to break them). |
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This is a suggestion for speeding up micros() a bit and a slight reduction in overall system interrupt latency.
There are a lot of comments added about the changes to help understand the intent. These may be cleaned up and/or removed as desired (if the pull request is accepted).
The 32-bit timer0_overflow_count has been renamed to timer0_micros and now counts in microseconds directly. The rename might be okay as no other parts of the core refer to it. However, since timer0_overflow_count was not static, it is possible that clients (i.e. sketches) are referring to it in which case the old name s/b kept.
With the change above, micros() no longer needs a 32-bit multiply to compute the result. Since the 32-bit counter is now directly in microseconds, all that is required is a 16-bit multiply of TCNT0 to add to the counter. This saves well over 20 instructions (call to 32-bit multiply library function) during execution, but the overall code is a few bytes larger.
micros() keeps interrupts disabled for a shorter length of time now, resulting in slightly improved overall
interrupt latency for other interrupts.