The phase delay is about 2.5mS. The hardware filter I use consists of a 15k resistor in series from the transformer secondary with a 10nF and 39k resistor in parallel to 0V. This drives a npn transistor the collector of which has a pullup resistor to 5V and drives the interrupt. I add/subtract a constant time value to correct for the phase shift. Most of the filtering is done in the software by a frequency and phase locked loop. In Australia some cities have a mains signalling technique called Zellweger. This is a large amplitude signal of a few khz injected onto the mains for off-peak power control. It is not synchronous with the mains and gives you bad zero crossing signals when it occurs in zero crossing time. This upsets most phase controlled dimmers. Thus the need for phase and frequency locking. also when people turn appliances on and off causes flashing of phase controlled lamps on many dimmers - the phase and frequency locking fixes this also.

For the technique of decoding manchester encoded signals see this:
http://clearwater.com.au/?action=code

As for sample code in assembler - you're out of luck. I wrote mine in 'c', but not the same way the writer of the above site used. Once you understand the technique the code is quite simple. I also used these techniques for receiving DALI encoded messages.