Documentation

The resetSynchronizer will synchronize an incoming reset according to whether the domain is synchronous or asynchronous.

For asynchronous resets this synchronizer ensures the reset will only be de-asserted synchronously but it can still be asserted asynchronously. The reset assert is immediate, but reset de-assertion is delayed by two cycles.

Normally, asynchronous resets can be both asynchronously asserted and de-asserted. Asynchronous de-assertion can induce meta-stability in the component which is being reset. To ensure this doesn't happen, resetSynchronizer ensures that de-assertion of a reset happens synchronously. Assertion of the reset remains asynchronous.

Note that asynchronous assertion does not induce meta-stability in the component whose reset is asserted. However, when a component "A" in another clock or reset domain depends on the value of a component "B" being reset, then asynchronous assertion of the reset of component "B" can induce meta-stability in component "A". To prevent this from happening you need to use a proper synchronizer, for example one of the synchronizers in Clash.Explicit.Synchronizer.

For synchronous resets this function ensures that the reset is asserted and de-asserted synchronously. Both the assertion and de-assertion of the reset are delayed by two cycles.

Example 1

topEntity
  :: Clock  System
  -> Reset  System
  -> Signal System Bit
  -> Signal System (BitVector 8)
topEntity clk asyncRst key1 =
  withClockResetEnable clk rst enableGen leds
 where
  rst   = resetSynchronizer clk asyncRst
  key1R = isRising 1 key1
  leds  = mealy blinkerT (1, False, 0) key1R

Example 2

topEntity
  :: Clock  System
  -> Reset  System
  -> Signal System Bit
  -> Signal System (BitVector 8)
topEntity clk rst key1 =
    let  (pllOut,pllStable) = altpll (SSymbol @"altpll50") clk rst
         rstSync            = resetSynchronizer pllOut (unsafeToHighPolarity pllStable)
    in   exposeClockResetEnable leds pllOut rstSync enableGen
  where
    key1R  = isRising 1 key1
    leds   = mealy blinkerT (1, False, 0) key1R

Implementation details

                                  rst
  --------------------------------------+
                      |                 |
                 +----v----+       +----v----+
    deasserted   |         |       |         |
  --------------->         +------->         +-------->
                 |         |       |         |
             +---|>        |   +---|>        |
             |   |         |   |   |         |
             |   +---------+   |   +---------+
     clk     |                 |
  -----------------------------+

                 +---------+       +---------+
    rst          |         |       |         |
  --------------->         +------->         +-------->
                 |         |       |         |
             +---|>        |   +---|>        |
             |   |         |   |   |         |
             |   +---------+   |   +---------+
     clk     |                 |
  -----------------------------+

Filter glitches from reset signals by only triggering a reset after it has been asserted for glitchlessPeriod cycles. It will then stay asserted for as long as the given reset was asserted consecutively.

If synthesized on a domain with initial values, resetGlitchFilter will output an asserted reset for glitchlessPeriod cycles (plus any cycles added by the given reset). If initial values can't be used, it will only output defined reset values after glitchlessPeriod cycles.

Example 1

>>> let sampleResetN n = sampleN n . unsafeToHighPolarity
>>> let resetFromList = unsafeFromHighPolarity . fromList
>>> let rst = resetFromList [True, True, False, False, True, False, False, True, True, False, True]
>>> sampleResetN 12 (resetGlitchFilter d2 systemClockGen rst)
[True,True,True,True,False,False,False,False,False,True,True,False]

Hold reset for a number of cycles relative to an incoming reset signal.

Example:

>>> let sampleWithReset = sampleN 8 . unsafeToHighPolarity
>>> sampleWithReset (holdReset @System clockGen enableGen (SNat @2) (resetGenN (SNat @3)))
[True,True,True,True,True,False,False,False]

holdReset holds the reset for an additional 2 clock cycles for a total of 5 clock cycles where the reset is asserted. holdReset also works on intermediate assertions of the reset signal:

>>> let rst = fromList [True, False, False, False, True, False, False, False]
>>> sampleWithReset (holdReset @System clockGen enableGen (SNat @2) (unsafeFromHighPolarity rst))
[True,True,True,False,True,True,True,False]

Convert between different types of reset, adding a synchronizer when the domains are not the same. See resetSynchronizer for further details about reset synchronization.

A reset signal belonging to a domain called dom.

The underlying representation of resets is Bool.

Reset generator

To be used like:

rstSystem = resetGen @System

See tbClockGen for example usage.

Generate reset that's asserted for the first n cycles.

To be used like:

rstSystem5 = resetGen System (SNat 5)

Example usage:

>>> sampleN 7 (unsafeToHighPolarity (resetGenN @System (SNat @3)))
[True,True,True,False,False,False,False]

Get ResetKind from a KnownDomain context. Example usage:

f :: forall dom . KnownDomain dom => ....
f a b c =
  case resetKind @dom of
    SAsynchronous -> foo
    SSynchronous -> bar

Reset generator for the System clock domain.

NB: should only be used for simulation or the testBench function.

Example

topEntity :: Vec 2 (Vec 3 (Unsigned 8)) -> Vec 6 (Unsigned 8)
topEntity = concat

testBench :: Signal System Bool
testBench = done
  where
    testInput      = pure ((1 :> 2 :> 3 :> Nil) :> (4 :> 5 :> 6 :> Nil) :> Nil)
    expectedOutput = outputVerifier' ((1:>2:>3:>4:>5:>6:>Nil):>Nil)
    done           = exposeClockResetEnable (expectedOutput (topEntity $ testInput)) clk rst
    clk            = tbSystemClockGen (not <$> done)
    rst            = systemResetGen

unsafeToReset is unsafe. For asynchronous resets it is unsafe because it can introduce combinatorial loops. In case of synchronous resets it can lead to meta-stability issues in the presence of asynchronous resets.

NB: You probably want to use unsafeFromLowPolarity or unsafeFromHighPolarity.

unsafeFromReset is unsafe because it can introduce:

• meta-stability

For asynchronous resets it is unsafe because it can cause combinatorial loops. In case of synchronous resets it can lead to meta-stability in the presence of asynchronous resets.

NB: You probably want to use unsafeToLowPolarity or unsafeToHighPolarity.

Convert a reset to an active high reset. Has no effect if reset is already an active high reset. Is unsafe because it can introduce:

• meta-stability

For asynchronous resets it is unsafe because it can cause combinatorial loops. In case of synchronous resets it can lead to meta-stability in the presence of asynchronous resets.

Convert a reset to an active low reset. Has no effect if reset is already an active low reset. It is unsafe because it can introduce:

• meta-stability

For asynchronous resets it is unsafe because it can cause combinatorial loops. In case of synchronous resets it can lead to meta-stability in the presence of asynchronous resets.

Interpret a signal of bools as an active high reset and convert it to a reset signal corresponding to the domain's setting.

For asynchronous resets it is unsafe because it can cause combinatorial loops. In case of synchronous resets it can lead to meta-stability in the presence of asynchronous resets.

Interpret a signal of bools as an active low reset and convert it to a reset signal corresponding to the domain's setting.

For asynchronous resets it is unsafe because it can cause combinatorial loops. In case of synchronous resets it can lead to meta-stability in the presence of asynchronous resets.