Add PulseIn Implementation

[stevemcquaid]

Add support for implementing the arduino pulseIn method and exposing that thru Firmata. Specifically needed for ultrasonic ranging sensors that use a pulse to measure distances. Polling the pin state over the network using a firmata client to take measurements is too noisy. The network latency is never going to be fast or stable enough to get any sort of accuracy for these timing-based sensors.

The time differential needed to compute measurements on these type of sensors, I believe, really needs to be done on the hardware itself. Network latency can add +-10 full milliseconds even under the best conditions. (Plus it would stink to have measurements change as a function of distance from wifi router.)

This code was taken from: @jgautier 's arduino fork,
Architectural approach from: @NeoPolus - pyFirmata#45
Relevant threads with gobot client:

• 361
• 152
• 246

[soundanalogous]

The problem with this implementation is it contains long blocking delays. This is fine if the only thing the user needs is pulsein support in Firmata for a given application, but if combining with other general features such as digital I/O, analog in, PWM, I2C, UART, etc the user may experience unexpected behavior due to the blocking delays. That is why I do not want to accept this PR for the general purpose. Come up with a non-blocking implementation and I will consider it.

[mircobabini]

Hi there! I need the same thing.

First question: is it good to use the 0x74 for the feature?

Second question: @soundanalogous are you referring to the delayMicroseconds usage? Due to the fact that Arduino is not multithreaded, how is it possible to make it non-blocking?