Question

I’m trying to implement temporal multiplexing to drive a 7-segment display with 4 digits: the device has 7 data legs and 4 anodes, so if you want to display four different digits, you have to set the anodes to 0001 first and the data legs to your segments; then after a while, set the anodes to 0010 and update the data legs; and so on.

I’m trying to implement this in Kansas Lava. However, the Xilinx compiler rejects the generated VHDL with a type error (and looking at the generated code, I think it’s right).

First, my Lava code: it basically implements a signal of the sequence [0, 1, 2, 3, 0, ...], then uses the .!. operator from Language.KansasLava.Signal to index into the matrix-of-matrices parameter. The anode value is generated by rotating 0001 at each timestep to the left.

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE DataKinds #-}
import Language.KansasLava
import Hardware.KansasLava.Boards.Papilio.LogicStart -- from http://github.com/gergoerdi/kansas-lava-papilio
import Data.Sized.Matrix
import Data.Sized.Unsigned as Unsigned
import Data.Bits

driveSS :: forall clk sig n. (Clock clk, sig ~ Signal clk, Size n, Rep n, Num n, Integral n) => Matrix n (Matrix X7 (sig Bool)) -> SevenSeg clk ActiveLow n
driveSS segss = SevenSeg (fmap bitNot anodes) segs high
  where
    clkAnode :: sig Bool
    clkAnode = divideClk (Witness :: Witness X8)

    selector :: sig n
    selector = counter clkAnode

    segss' :: sig (Matrix n (Matrix X7 Bool))
    segss' = pack . fmap pack $ segss

    segs :: Matrix X7 (sig Bool)
    segs = unpack $ segss' .!. selector

    anodes :: Matrix n (sig Bool)
    anodes = rotatorL clkAnode

test_sseg :: Fabric ()
test_sseg = do
    sw <- switches
    let sw' = cropAt sw 1
    sseg $ driveSS $ matrix [sw', zero, zero, zero]
  where
    zero = matrix $ replicate 7 low

divideClk :: forall c sig ix. (Clock c, sig ~ Signal c, Size ix) => Witness ix -> sig Bool
divideClk _ = counter high .==. (0 :: sig (Unsigned ix))

counter :: (Rep a, Num a, Clock c, sig ~ Signal c) => sig Bool -> sig a
counter inc = loop
  where
    reg = register 0 loop
    loop = mux inc (reg, reg + 1)

rotatorL :: (Clock c, sig ~ Signal c, Size ix, Integral ix) => sig Bool -> Matrix ix (sig Bool)
rotatorL step = fromUnsigned loop
  where
    reg = register 1 loop
    loop = mux step (reg, rotateL reg 1)

fromUnsigned :: (sig ~ Signal c, Size ix) => sig (Unsigned ix) -> Matrix ix (sig Bool)
fromUnsigned = unpack . coerce Unsigned.toMatrix

main :: IO ()
main = do
    writeVhdlPrelude "lava-prelude.vhdl"
    kleg <- reifyFabric $ do
        board_init
        test_sseg
    writeVhdlCircuit "hello" "hello.vhdl" kleg
    writeUCF "hello.ucf" kleg

So when I try to compile the generated VHDL, I get this error message:

ERROR:HDLParsers:800 - "/home/cactus/prog/lava/hello/src/hello.vhdl" Line 85. Type of sig_24_o0 is incompatible with type of sig_28_o0.

The relevant lines from hello.vhdl are:

type sig_24_o0_type is array (7 downto 0) of std_logic_vector(0 downto 0);
signal sig_24_o0 : sig_24_o0_type;

signal sig_25_o0 : std_logic_vector(1 downto 0);

type sig_28_o0_type is array (3 downto 0) of std_logic_vector(6 downto 0);
signal sig_28_o0 : sig_28_o0_type;

sig_24_o0 <= sig_28_o0(to_integer(unsigned(sig_25_o0)));

The type of sig_24_o0 seems wrong; I think it should be either array (6 downto 0) of std_logic_vector(0 downto 0) or std_logic_vector(6 downto 0), but I don’t know what Lava uses those std_logic_vector(0 downto 0)‘s.

Answer 1

I ended up working around this by multiplexing per-wire instead of multiplexing the whole bus:

segss' :: Matrix X7 (Matrix n (sig Bool))
segss' = columns . joinRows $ segss

segs :: Matrix X7 (sig Bool)
segs = fmap (nary selector) segss'

using the helper function

nary :: forall a clk sig n. (Clock clk, sig ~ Signal clk, Rep a, Size n, Rep n) => sig n -> Matrix n (sig a) -> sig a
nary sel inps = pack inps .!. sel

The VHDL generated by this compiles just fine; although I have no idea if it makes the resulting circuit any more complicated (or maybe even simpler).