add pybNano pi demo

Author: shaoziyang

boards/pybNano/pi/upi.py

@@ -0,0 +1,31 @@
+'''
+
+    pybNano board
+    pi calculate demo
+
+'''
+
+from utime import ticks_ms, ticks_diff
+
+def pi(places=1000):
+  # 3 + 3*(1/24) + 3*(1/24)*(9/80) + 3*(1/24)*(9/80)*(25/168)
+  # The numerators 1, 9, 25, ... are given by (2x + 1) ^ 2
+  # The denominators 24, 80, 168 are given by (16x^2 -24x + 8)
+  extra = 8
+  one = 10 ** (places+extra)
+  t, c, n, na, d, da = 3*one, 3*one, 1, 0, 0, 24
+
+  while t > 1: 
+    n, na, d, da = n+na, na+8, d+da, da+32
+    t = t * n // d
+    c += t
+  return c // (10 ** extra)
+
+def pi_t(n=1000):
+    t1 = ticks_ms()
+    r = pi(n)
+    t2 = ticks_ms()
+    print('Calculation time:', ticks_diff(t2, t1), 'ms')
+    print('Result:', r)
+
+pi_t(1000)