CLaSH.Prelude.Synchronizer

Synchronizer circuits for safe clock domain crossings

Bit-synchronizers

dualFlipFlopSynchronizer

Arguments

:: SClock clk1

Clock to which the incoming data is synchronised

-> SClock clk2

Clock to which the outgoing data is synchronised

-> a

Initial value of the two synchronisation registers

-> Signal' clk1 a

Incoming data

-> Signal' clk2 a

Outgoing, synchronised, data

Synchroniser based on two sequentially connected flip-flops.

• NB: This synchroniser can be used for bit-synchronization.

• NB: Although this synchroniser does reduce metastability, it does not guarantee the proper synchronisation of a whole word. For example, given that the output is sampled twice as fast as the input is running, and we have two samples in the input stream that look like:

  [0111,1000]

  But the circuit driving the input stream has a longer propagation delay on msb compared to the lsbs. What can happen is an output stream that looks like this:

  [0111,0111,0000,1000]

  Where the level-change of the msb was not captured, but the level change of the lsbs were.

  If you want to have safe word-synchronisation use asyncFIFOSynchronizer.

Word-synchronizers

asyncFIFOSynchronizer

Arguments

:: 2 <= addrSize

=> SNat addrSize

Size of the internally used addresses, the FIFO contains 2^addrSize elements.

-> SClock wclk

Clock to which the write port is synchronised

-> SClock rclk

Clock to which the read port is synchronised

-> Signal' rclk Bool

Read request

-> Signal' wclk (Maybe a)

Element to insert

-> (Signal' rclk a, Signal' rclk Bool, Signal' wclk Bool)

(Oldest element in the FIFO, empty flag, full flag)

Synchroniser implemented as a FIFO around an asynchronous RAM. Based on the design described in CLaSH.Tutorial, which is itself based on the design described in http://www.sunburst-design.com/papers/CummingsSNUG2002SJ_FIFO1.pdf.

NB: This synchroniser can be used for word-synchronization.