From fc1959bd9887ecc4d4ceb62a53e87abc6f49ef00 Mon Sep 17 00:00:00 2001
From: Nathan Ringo <nathan@remexre.com>
Date: Mon, 23 Sep 2024 21:46:34 -0500
Subject: Adds README, moves fpga stuff to a subdirectory.

---
 fpga/src/Uart.bs | 148 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 fpga/src/Uart.bs

(limited to 'fpga/src/Uart.bs')

diff --git a/fpga/src/Uart.bs b/fpga/src/Uart.bs
new file mode 100644
index 0000000..1059a65
--- /dev/null
+++ b/fpga/src/Uart.bs
@@ -0,0 +1,148 @@
+package Uart where
+
+import FIFOF
+import GetPut
+import Util
+
+interface Clock =
+  clk :: Bool
+
+mkDivider :: Integer -> Module Clock
+mkDivider divisor =
+  module
+    count :: Reg (Bit 32) <- mkReg 0
+
+    rules
+      "increment_divider": when True ==> do
+        if count == fromInteger (divisor - 1) then do
+          count := 0
+        else
+          count := count + 1
+
+    interface Clock
+      clk = count == 0
+
+-- | The state of the TX side of the UART.
+data TxState
+  = -- | The UART is not currently sending anything. May transition to
+    -- 'Start b' when ready to send 'b'.
+    Idle
+  | -- | The UART is about to send the start bit. 'Start b' transitions to
+    -- 'Data b 7' by sending the start bit.
+    Start (Bit 8)
+  | -- | The UART is about to send a data bit. 'Data b n' transitions to
+    -- 'Data (b >> 1) (n - 1)' by sending a data bit. 'Data b 0' transitions to
+    -- 'Idle' by sending the last data bit. Being in the 'Idle' state for a
+    -- clock transmits the stop bit.
+    Data (Bit 8) (Bit 3)
+  deriving (Bits)
+
+-- | The TX side of the UART.
+interface TxUart =
+  -- | The TX pin.
+  pin :: Bit 1
+  -- | Writes a byte to the UART's transmit buffer.
+  send :: Put (Bit 8)
+
+mkTxUart :: Clock -> Integer -> Module TxUart
+mkTxUart baudClock bufferSize =
+  module
+    fifo :: FIFOF (Bit 8) <- mkSizedFIFOF bufferSize
+    state :: Reg TxState <- mkReg Idle
+    pin :: Reg (Bit 1) <- mkReg 1
+
+    rules
+      "uart_tx": when baudClock.clk
+        rules
+          "uart_tx_idle": when Idle <- state, not fifo.notEmpty ==> do
+            pin := 1
+          "uart_tx_idle_to_start": when Idle <- state, fifo.notEmpty ==> do
+            pin := 1
+            b <- (toGet fifo).get
+            state := Start b
+          "uart_tx_start": when Start b <- state ==> do
+            pin := 0
+            state := Data b 7
+          "uart_tx_data": when Data b n <- state ==> do
+            pin := b[0:0]
+            if n == 0 then
+              state := Idle
+            else
+              state := Data (b >> 1) (n - 1)
+    interface TxUart
+      pin = pin
+      send = toPut fifo
+
+-- | The state of the RX side of the UART.
+data RxState
+  = -- | The initial state of the UART, and the state after receiving the stop
+    -- bit. May transition to 'Data 0 0' when the start bit is received.
+    Idle
+  | -- | In the 'Data _ n' state, the UART has received the start bit and 'n'
+    -- data bits, and is about to receive more data bits. 'Data _ n'
+    -- transitions to 'Data _ (n + 1)' by receiving a data bit. 'Data b 7'
+    -- transitions to 'Stop' by receving the last data bit.
+    Data (Bit 8) (Bit 3)
+  | -- | In the 'Stop' state, the UART has received the start and data bits,
+    -- and is waiting for the stop bit (which is ignored). Transitions to
+    -- 'Idle'.
+    Stop (Bit 8)
+  deriving (Bits, FShow)
+
+-- | The RX side of the UART.
+interface RxUart =
+  -- | The RX pin.
+  pin :: Bit 1 -> Action
+  -- | Reads a byte from the UART's receive buffer.
+  recv :: Get (Bit 8)
+
+mkRxUart :: Clock -> Integer -> Module RxUart
+mkRxUart baudClock bufferSize =
+  module
+    fifo :: FIFOF (Bit 8) <- mkGSizedFIFOF True False bufferSize
+    state :: Reg RxState <- mkReg Idle
+    pin :: Wire (Bit 1) <- mkWire
+
+    rules
+      "uart_rx": when baudClock.clk
+        rules
+          "uart_rx_idle_to_start": when Idle <- state, pin == 0 ==> do
+            state := Data 0 0
+          "uart_rx_data_to_data": when Data bits n <- state, n < 7 ==> do
+            state := Data (pin ++ bits[7:1]) (n + 1)
+          "uart_rx_data_to_stop": when Data bits 7 <- state ==> do
+            state := Stop (pin ++ bits[7:1])
+          "uart_rx_stop_to_idle": when Stop bits <- state, pin == 1 ==> do
+            fifo.enq bits
+            state := Idle
+
+    interface RxUart
+      pin bit = pin := bit
+      recv = toGet fifo
+
+-- | An 8n1 UART.
+interface Uart =
+  -- | The RX pin.
+  rxPin :: Bit 1 -> Action
+  -- | The TX pin.
+  txPin :: Bit 1
+
+  -- | Reads a byte from the UART's receive buffer.
+  recv :: Get (Bit 8)
+  -- | Writes a byte to the UART's transmit buffer.
+  send :: Put (Bit 8)
+
+mkUart :: Integer -> Module Uart
+mkUart baudDivisor =
+  module
+    baudClock <- mkDivider baudDivisor
+    rx <- mkRxUart baudClock 8
+    tx <- mkTxUart baudClock 8
+
+    interface Uart
+      rxPin = rx.pin
+      txPin = tx.pin
+      recv = rx.recv
+      send = tx.send
+
+-- vim: set ft=haskell :
-- 
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