Air packet에서 Bluetooth clock 확인

 

 

Clock delta : 0x0c (12)

Time delta : 003750

Calc : 12  * 625 / 2

 

Clock이 625/2 인 이유는 625μs를 확인하기 위해서는 최소한 2배의 속도로 체크해야 하기 때문이다.

 

 

1.1 BLUETOOTH CLOCK
Every Bluetooth device shall have a native clock that shall be derived from a
free running reference clock. Offsets may be added to the reference clock to
synchronize the native clock with other non-Bluetooth systems. For
synchronization with other Bluetooth devices, offsets are used that, when
added to the native clock, provide temporary Bluetooth clocks that are mutually
synchronized. It should be noted that the Bluetooth clock has no relation to the
time of day; it may therefore be initialized to any value. The clock has a cycle of
about a day. If the clock is implemented with a counter, a 28-bit counter is
required that shall wrap around at 228-1. The least significant bit (LSB) shall
tick in units of 312.5 μs (i.e. half a time slot), giving a clock rate of 3.2 kHz.
The clock determines critical periods and triggers the events in the device.
Four periods are important in the Bluetooth system: 312.5 μs, 625 μs, 1.25 ms,
and 1.28 s; these periods correspond to the timer bits CLK0, CLK1, CLK2, and
CLK12, respectively, see Figure 1.4 on page 68.

 

Bluetooth Air에서 기본으로 사용하는 시간 단위는 625 μs.

 

 

2.2.5.1 Piconet physical channel timing
In the figures, only single-slot packets are shown as an example.
The master TX/RX timing is shown in Figure 2.3 on page 76. In Figure 2.3 and
Figure 2.4 the channel hopping frequencies are indicated by f(k) where k is the
time slot number. After transmission, a return packet is expected N × 625 μs
after the start of the TX packet where N is an odd, integer larger than 0. N
depends on the type of the transmitted packet.
To allow for some time slipping, an uncertainty window is defined around the
exact receive timing. During normal operation, the window length shall be 20
μs, which allows the RX packet to arrive up to 10 μs too early or 10 μs too late.
It is recommended that slaves implement variable sized windows or time
tracking to accommodate a master's absence of more than 250ms.
During the beginning of the RX cycle, the access correlator shall search for the
correct channel access code over the uncertainty window. If an event trigger
does not occur the receiver may go to sleep until the next RX event. If in the
course of the search, it becomes apparent that the correlation output will never
exceed the final threshold, the receiver may go to sleep earlier. If a trigger
event occurs, the receiver shall remain open to receive the rest of the packet
unless the packet is for another device, a non-recoverable header error is
detected, or a non-recoverable payload error is detected.

 

Bluetooth에서는 clock에 대한 시간 오차를 기준에서 앞/뒤 10μs 까지는 허용한다.

즉, 기준에서 10μs를 벗어나면 안 된다.