Clash.Examples

Decoders and Encoders

Decoder

Using a case statement:

decoderCase :: Bool -> BitVector 4 -> BitVector 16
decoderCase enable binaryIn | enable =
  case binaryIn of
    0x0 -> 0x0001
    0x1 -> 0x0002
    0x2 -> 0x0004
    0x3 -> 0x0008
    0x4 -> 0x0010
    0x5 -> 0x0020
    0x6 -> 0x0040
    0x7 -> 0x0080
    0x8 -> 0x0100
    0x9 -> 0x0200
    0xA -> 0x0400
    0xB -> 0x0800
    0xC -> 0x1000
    0xD -> 0x2000
    0xE -> 0x4000
    0xF -> 0x8000
decoderCase _ _ = 0

Using the shiftL function:

decoderShift :: Bool -> BitVector 4 -> BitVector 16
decoderShift enable binaryIn =
  if enable
     then 1 `shiftL` (fromIntegral binaryIn)
     else 0

Examples:

>>> decoderCase True 3
0000_0000_0000_1000
>>> decoderShift True 7
0000_0000_1000_0000

The following property holds:

\enable binaryIn -> decoderShift enable binaryIn === decoderCase enable binaryIn

Encoder

Using a case statement:

encoderCase :: Bool -> BitVector 16 -> BitVector 4
encoderCase enable binaryIn | enable =
  case binaryIn of
    0x0001 -> 0x0
    0x0002 -> 0x1
    0x0004 -> 0x2
    0x0008 -> 0x3
    0x0010 -> 0x4
    0x0020 -> 0x5
    0x0040 -> 0x6
    0x0080 -> 0x7
    0x0100 -> 0x8
    0x0200 -> 0x9
    0x0400 -> 0xA
    0x0800 -> 0xB
    0x1000 -> 0xC
    0x2000 -> 0xD
    0x4000 -> 0xE
    0x8000 -> 0xF
encoderCase _ _ = 0

The following property holds:

\en decIn -> en ==> (encoderCase en (decoderCase en decIn) === decIn)

Counters

8-bit Simple Up Counter

Using register:

upCounter
  :: HiddenClockReset domain gated synchronous
  => Signal domain Bool
  -> Signal domain (Unsigned 8)
upCounter enable = s
  where
    s = register 0 (mux enable (s + 1) s)

8-bit Up Counter With Load

Using mealy:

upCounterLd
  :: HiddenClockReset domain gated synchronous
  => Signal domain (Bool,Bool,Unsigned 8)
  -> Signal domain (Unsigned 8)
upCounterLd = mealy upCounterLdT 0

upCounterLdT s (ld,en,dIn) = (s',s)
  where
    s' | ld        = dIn
       | en        = s + 1
       | otherwise = s

8-bit Up-Down counter

Using register and mux:

upDownCounter
  :: HiddenClockReset domain gated synchronous
  => Signal domain Bool
  -> Signal domain (Unsigned 8)
upDownCounter upDown = s
  where
    s = register 0 (mux upDown (s + 1) (s - 1))

The following property holds:

\en -> en ==> testFor 1000 (upCounter (pure en) .==. upDownCounter (pure en))

LFSR

External/Fibonacci LFSR, for n=16 and using the primitive polynominal 1 + x^11 + x^13 + x^14 + x^16

lfsrF' :: BitVector 16 -> BitVector 16
lfsrF' s = pack feedback ++# slice d15 d1 s
  where
    feedback = s!5 `xor` s!3 `xor` s!2 `xor` s!0

lfsrF
  :: HiddenClockReset domain gated synchronous
  => BitVector 16
  -> Signal domain Bit
lfsrF seed = msb <$> r
  where r = register seed (lfsrF' <$> r)

We can also build a internal/Galois LFSR which has better timing characteristics. We first define a Galois LFSR parametrizable in its filter taps:

lfsrGP taps regs = zipWith xorM taps (fb +>> regs)
  where
    fb  = last regs
    xorM i x | i         = x `xor` fb
             | otherwise = x

Then we can instantiate a 16-bit LFSR as follows:

lfsrG :: HiddenClockReset domain gated synchronous => BitVector 16 -> Signal domain Bit
lfsrG seed = last (unbundle r)
  where r = register (unpack seed) (lfsrGP (unpack 0b0011010000000000) <$> r)

The following property holds:

testFor 100 (lfsrF 0xACE1 .==. lfsrG 0x4645)

Gray counter

Using the previously defined upCounter:

grayCounter
  :: HiddenClockReset domain gated synchronous
  => Signal domain Bool
  -> Signal domain (BitVector 8)
grayCounter en = gray <$> upCounter en
  where gray xs = pack (msb xs) ++# xor (slice d7 d1 xs) (slice d6 d0 xs)

One-hot counter

Basically a barrel-shifter:

oneHotCounter
  :: HiddenClockReset domain gated synchronous
  => Signal domain Bool
  -> Signal domain (BitVector 8)
oneHotCounter enable = s
  where
    s = register 1 (mux enable (rotateL <$> s <*> 1) s)

Parity and CRC

Parity

Just reduceXor:

parity :: Unsigned 8 -> Bit
parity data_in = reduceXor data_in

Serial CRC

• Width = 16 bits
• Truncated polynomial = 0x1021
• Initial value = 0xFFFF
• Input data is NOT reflected
• Output CRC is NOT reflected
• No XOR is performed on the output CRC

crcT bv dIn = replaceBit 0  dInXor
            $ replaceBit 5  (bv!4  `xor` dInXor)
            $ replaceBit 12 (bv!11 `xor` dInXor)
              rotated
  where
    dInXor  = dIn `xor` fb
    rotated = rotateL bv 1
    fb      = msb bv

crc
  :: HiddenClockReset domain gated synchronous
  => Signal domain Bool
  -> Signal domain Bool
  -> Signal domain Bit
  -> Signal domain (BitVector 16)
crc enable ld dIn = s
  where
    s = register 0xFFFF (mux enable (mux ld 0xFFFF (crcT <$> s <*> dIn)) s)

UART model

{-# LANGUAGE RecordWildCards #-}
module UART (uart) where

import Clash.Prelude
import Control.Lens
import Control.Monad
import Control.Monad.Trans.State

-- UART RX Logic
data RxReg
  = RxReg
  { _rx_reg        :: BitVector 8
  , _rx_data       :: BitVector 8
  , _rx_sample_cnt :: Unsigned 4
  , _rx_cnt        :: Unsigned 4
  , _rx_frame_err  :: Bool
  , _rx_over_run   :: Bool
  , _rx_empty      :: Bool
  , _rx_d1         :: Bit
  , _rx_d2         :: Bit
  , _rx_busy       :: Bool
  }

makeLenses ''RxReg

uartRX r@(RxReg {..}) rx_in uld_rx_data rx_enable = flip execState r $ do
  -- Synchronise the async signal
  rx_d1 .= rx_in
  rx_d2 .= _rx_d1
  -- Uload the rx data
  when uld_rx_data $ do
    rx_data  .= _rx_reg
    rx_empty .= True
  -- Receive data only when rx is enabled
  if rx_enable then do
    -- Check if just received start of frame
    when (not _rx_busy && _rx_d2 == 0) $ do
      rx_busy       .= True
      rx_sample_cnt .= 1
      rx_cnt        .= 0
    -- Star of frame detected, Proceed with rest of data
    when _rx_busy $ do
      rx_sample_cnt += 1
      -- Logic to sample at middle of data
      when (_rx_sample_cnt == 7) $ do
        if _rx_d1 == 1 && _rx_cnt == 0 then
          rx_busy .= False
        else do
          rx_cnt += 1
          -- start storing the rx data
          when (_rx_cnt > 0 && _rx_cnt < 9) $ do
            rx_reg %= replaceBit (_rx_cnt - 1) _rx_d2
          when (_rx_cnt == 9) $ do
            rx_busy .= False
            -- Check if End of frame received correctly
            if _rx_d2 == 0 then
              rx_frame_err .= True
            else do
              rx_empty     .= False
              rx_frame_err .= False
              -- Check if last rx data was not unloaded
              rx_over_run  .= not _rx_empty
  else do
    rx_busy .= False

-- UART TX Logic
data TxReg
  = TxReg
  { _tx_reg      :: BitVector 8
  , _tx_empty    :: Bool
  , _tx_over_run :: Bool
  , _tx_out      :: Bit
  , _tx_cnt      :: Unsigned 4
  }

makeLenses ''TxReg

uartTX t@(TxReg {..}) ld_tx_data tx_data tx_enable = flip execState t $ do
  when ld_tx_data $ do
    if not _tx_empty then
      tx_over_run .= False
    else do
      tx_reg   .= tx_data
      tx_empty .= False
  when (tx_enable && not _tx_empty) $ do
    tx_cnt += 1
    when (_tx_cnt == 0) $
      tx_out .= 0
    when (_tx_cnt > 0 && _tx_cnt < 9) $
      tx_out .= _tx_reg ! (_tx_cnt - 1)
    when (_tx_cnt == 9) $ do
      tx_out   .= 1
      tx_cnt   .= 0
      tx_empty .= True
  unless tx_enable $
    tx_cnt .= 0

-- Combine RX and TX logic
uart ld_tx_data tx_data tx_enable rx_in uld_rx_data rx_enable =
    ( _tx_out   <$> txReg
    , _tx_empty <$> txReg
    , _rx_data  <$> rxReg
    , _rx_empty <$> rxReg
    )
  where
    rxReg     = register rxRegInit (uartRX <$> rxReg <*> rx_in <*> uld_rx_data
                                           <*> rx_enable)
    rxRegInit = RxReg { _rx_reg        = 0
                      , _rx_data       = 0
                      , _rx_sample_cnt = 0
                      , _rx_cnt        = 0
                      , _rx_frame_err  = False
                      , _rx_over_run   = False
                      , _rx_empty      = True
                      , _rx_d1         = 1
                      , _rx_d2         = 1
                      , _rx_busy       = False
                      }

    txReg     = register txRegInit (uartTX <$> txReg <*> ld_tx_data <*> tx_data
                                           <*> tx_enable)
    txRegInit = TxReg { _tx_reg      = 0
                      , _tx_empty    = True
                      , _tx_over_run = False
                      , _tx_out      = 1
                      , _tx_cnt      = 0
                      }