Add iCESugar-nano board

Follow the 6 steps to add support for a new target.

Step 1: Locate input and output pins.

Step 2: Add pin constraint file. This covers clock input, LED and UART
        output.

Step 3: Create a clock generator. The chip has no internal clock
        generator so the external 12 MHz clock is used.

Step 4: Add top level servant_ice40_cm36 which connects the one-wire
        output of servant to the LED pin.

Step 5: Add fileset including the new top level and pin constraints.

Step 6: Add target icesugar-nano.

data/icesugar_nano.pcf

@@ -0,0 +1,9 @@
+# 12 MHz external clock
+set_io i_clk        D1
+
+# LED
+set_io o_led        B6
+
+# UART
+set_io i_uart_rx    A3
+set_io o_uart_tx    B3

doc/servant.rst

@@ -155,6 +155,19 @@ serial console will show up.
 
     fusesoc run --target=icesugar servant
 
+iCESugar-nano
+^^^^^^^^^^^^^
+
+Pin B3 is used for LED output. As the default clock of 12 MHz is rather slow
+the LED only toggles every 9 seconds with the default blinky example.
+
+Thanks to the onboard debugger, you can just connect the USB Type-C connector
+to the PC, and a serial console will show up. However, the device doesn't have
+enough RAM to run the Zephyr hello-world example so the UART pins are not
+connect but they are defined in the PCF for easy reference.
+
+    fusesoc run --target=icesugar-nano servant
+
 ICE-V Wireless
 ^^^^^^^^^^^^^^
 

servant.core

@@ -157,6 +157,11 @@ filesets:
 
   icesugar   : {files: [data/icesugar.pcf : {file_type : PCF}]}
 
+  icesugar_nano:
+    files:
+      - data/icesugar_nano.pcf : {file_type : PCF}
+      - servant/servant_ice40_cm36.v : {file_type : verilogSource}
+
   icev_wireless : {files: [data/icev_wireless.pcf  : {file_type : PCF}]}
 
   gmm7550:
@@ -441,6 +446,17 @@ targets:
         pnr: next
     toplevel : service
 
+  icesugar-nano:
+    default_tool : icestorm
+    description : iCE40LP1K Development Board by MuseLab
+    filesets : [mem_files, soc, icesugar_nano]
+    parameters : [memfile=blinky.hex, memsize=7168]
+    tools:
+      icestorm:
+        nextpnr_options: [--lp1k, --package, cm36, --freq, 12]
+        pnr: next
+    toplevel : servant_ice40_cm36
+
   icev_wireless:
     default_tool : icestorm
     description: ICE-V Wireless

servant/servant_ice40_cm36.v

@@ -0,0 +1,41 @@
+`default_nettype none
+module servant_ice40_cm36 (
+    input  wire i_clk,
+    output wire o_led
+);
+
+  parameter memfile = "blinky.hex";
+  parameter memsize = 7168;
+
+  wire wb_clk;
+  reg  wb_rst;
+
+  /* iCE40 CM36 has no PLL. Drive everything from the external clock. */
+  assign wb_clk = i_clk;
+
+  /* Board has no button that can be used for reset, but blinky doesn't
+   * work at all if the reset isn't enabled for at least 25 clocks.
+   *
+   * This will generate a reset signal at power on.
+   */
+  reg [7:0] rst_cnt = '0;
+
+  always @(posedge i_clk) begin
+    if (rst_cnt < 255) begin
+      rst_cnt <= rst_cnt + 1;
+      wb_rst  <= 1;
+    end else begin
+      wb_rst <= 0;
+    end
+  end
+
+  servant #(
+      .memfile(memfile),
+      .memsize(memsize)
+  ) servant (
+      .wb_clk(wb_clk),
+      .wb_rst(wb_rst),
+      .q     (o_led)
+  );
+
+endmodule