I am about to install a hall effect flow meter into my mains line, my sensor is rated 1-120 l/m with Q=4.5. Therefore, at full flow it will produce 540 pulses per second, in excess of the maximum of 200 stated by Ray. I see a few options to solving this:
1. Assume the flow wont go above ~45 l/m (poor option)
2. Modify the firmware to allow 540 pulses per second
3. Introduce hardware to reduce the pulse rate by some factor
Option 2 is likely the easiest, although changes like this always have downstream maintenance issues with future firmware updates. The micro would also have to deal with 540 interrupts per second, although the isr overhead would be low since all it does is increment a counter.
I think option 3 might be the best solution here with a divisor of 27 to give 10 pulses per litre and a maximum pulse rate of 20/sec. A regulator, Attiny85 and transistor should fit the bill here nicely. Configuring the transistor as an open-drain allows for voltage level matching using pin1 of the flow sensor on OpenSprinkler with its built in pull-up resistor. This setup might even fit into the head of the flow meter, or perhaps as a daughter board within OpenSprinkler.
Anyone have any comments?