helloworld_ecp5: add delays between messages

Otherwise the FTDI UART goes out of sync and corrupts data.

examples/helloworld_ecp5.py

@@ -22,17 +22,26 @@ class Top(Elaboratable):
                      init=[ord(c) for c in string])
         m.submodules.rdport = rdport = mem.read_port(synchronous=False)
 
+        wait = Signal()
+
         tx = uart.RS232TX(round(2**32*self.baudrate/100e6))
         m.submodules.tx = tx
         m.d.comb += [
-            tx.stb.eq(1),
+            tx.stb.eq(~wait),
             tx.data.eq(rdport.data),
             self.serial_tx.eq(tx.tx)
         ]
 
-        with m.If(tx.ack):
+        release = Signal()
+        counter = Signal(25)
+        m.d.sync += Cat(counter, release).eq(counter + 1)
+        with m.If(release):
+            m.d.sync += wait.eq(0)
+
+        with m.If(~wait & tx.ack):
             with m.If(rdport.addr == len(string) - 1):
                 m.d.sync += rdport.addr.eq(0)
+                m.d.sync += wait.eq(1)
             with m.Else():
                 m.d.sync += rdport.addr.eq(rdport.addr + 1)