Staying in step

No clock wire

No wire tells the devices when to read. Each counts time with its own crystal, and all are set to the same bit rate. Two crystals at the same rate are never exactly the same. The original specification allows an oscillator to be off by about half a per cent, and a later change in it about one and a half, which is why devices use crystals and not simple RC oscillators.

Two devices one per cent apart slide a whole bit apart in a hundred bits, and then one reads a bit the other has left.

The first edge sets everyone together

A device with nothing to send waits for the rail to go from quiet to loud. That edge is the start of frame, and every waiting device restarts its bit count on it. This is hard synchronisation. It does not make the crystals equal, only the counts, at the moment that matters.

A device does not read at the edge, because the rail is still changing there. It reads at the sample point, about 75 to 87 per cent of the way through the bit. No single number is the rule, and ESP-IDF's ready-made 500 kbit/s timing for the ESP32 uses 80 per cent.

Later edges nudge

Each later quiet-to-loud edge that arrives a little early or late tells a device its count has drifted. It lengthens or shortens its current bit by a small amount, never more than a set jump, at most once per bit. This is resynchronisation, and it holds the devices together to the end of the message.

It needs edges to keep coming. The longest gap the specification counts on is 29 bit times, and lesson eight shows how a message makes sure of it.

The rail takes time

A signal covers about five nanoseconds a metre, so the far end sees every edge late. A sender must still hear another device's loud bit before its own sample point, which limits how long a rail can be at a given speed. Lesson eleven comes back to it.

Common mistakes

• Looking for the clock. There is none. Each device has its own crystal and recovers the timing from the edges.
• Treating the sample point as one fixed number. It is a setting, about 75 to 87 per cent of the bit.
• Setting two devices to different bit rates and expecting synchronisation to cope. It corrects a small drift at the same rate. A device at another rate sees edges in the wrong places and reports errors.