Homelink is a standard for wireless remote activation. It is usually used to remotely open garage doors and gates. Most modern vehicles include a homelink remote in the car somewhere (either as an option, standard equipment, or OEM addition).
As you approach your gate, you press one of three homelink buttons, and the gate conveniently opens to allow your vehicle in. The problem is one of range. When you press the button it often works for only a second or so - if the homelink receiver is not in range, the gate will not open, and you need to drive closer before trying again.
When reverse-engineering the Tesla Roadster homelink module, we found the CAN bus message to activate homelink, and just assumed that would send the homelink 'open' message and then quickly stop. Turns out we were wrong. Timothy, on the Tesla Motors Club forums, first noticed the problem:
What I have noticed is this: when I press the Homelink button on the car and I am too far away from the gate, the gate doesn't open. When I push the Homelink button on the OVMS from too far away, the gate doesn't open either. BUT when I continue driving towards the gate, it then opens without pushing the Homelink button a second time. I am within line of sight of the gate the entire time. When I try this on my garage door, I push the OVMS button about a hundred yards away from the garage. I am down a hill and around a corner from the garage--completely out of sight. When I turn the bend and crest the hill and am in sight of the garage--20 yards away, the door then opens without pushing the button again. This does not happen when I push the Homelink button in the car that far away. For the garage door to open with the Homelink button on the car, I have to be 20 yards away and in line of sight. And either way, the garage door can be opened from 20 yards away, but the gate only 5 yards away. A big but consistent difference in distance. So I don't really have longer range with the OVMS, I can just push the button further away from the gate/garage. But the gate/garage starts opening the same distance from the car whether I use the OVMS or car Homelink button. The OVMS just seems a lot cooler!
There are times when a bug becomes very useful. It turns out that the Homelink module in the Tesla Roadster will eventually time-out the homelink command and stop transmitting the 'open' command, but that will be after a much longer time than usual. We could stop the transmission, but it is so much more useful not to.
With the latest OVMS code, we've even taken this to the next level of convenience by introducing geo fencing. Now, when you drive towards your home, you can configure OVMS to automatically activate Homelink as you approach. The gate opens, with no button presses required.