applet.interface.uart: only report baud updates when outside a +/- 5%…

… window

Because we are now updating the bit time whenever it changes, it is
possible to get a lot of "switched to __ baud" messages, even if the
change is minor and well within tolerance.

This patch will only report baudrate changes when they exceed a +/- 5%
window vs the previously reported baudrate.

software/glasgow/applet/interface/uart/__init__.py

@@ -260,7 +260,11 @@ async def _monitor_errors(self, device):
                 self.logger.warning("%d frame or parity errors detected", delta)
 
             new_bit_cyc = await device.read_register(self.__addr_bit_cyc, width=4)
-            if new_bit_cyc != cur_bit_cyc:
+
+            cur_bit_cyc_lo = cur_bit_cyc * 0.95
+            cur_bit_cyc_hi = cur_bit_cyc * 1.05
+
+            if new_bit_cyc < cur_bit_cyc_lo or new_bit_cyc > cur_bit_cyc_hi:
                 self.logger.info("switched to %d baud",
                                  self.__sys_clk_freq // (new_bit_cyc + 1))
             cur_bit_cyc = new_bit_cyc