add configurable time between NTP calls

README.rst

@@ -70,7 +70,7 @@ Usage Example
     wifi.radio.connect(wifi_ssid, wifi_password)
 
     pool = adafruit_connection_manager.get_radio_socketpool(wifi.radio)
-    ntp = adafruit_ntp.NTP(pool, tz_offset=0)
+    ntp = adafruit_ntp.NTP(pool, tz_offset=0, cache_seconds=3600)
 
     while True:
         print(ntp.datetime)

adafruit_ntp.py

@@ -46,6 +46,7 @@ def __init__(
         port: int = 123,
         tz_offset: float = 0,
         socket_timeout: int = 10,
+        cache_seconds: int = 0,
     ) -> None:
         """
         :param object socketpool: A socket provider such as CPython's `socket` module.
@@ -55,6 +56,8 @@ def __init__(
             CircuitPython. CPython will determine timezone automatically and adjust (so don't use
             this.) For example, Pacific daylight savings time is -7.
         :param int socket_timeout: UDP socket timeout, in seconds.
+        :param int cache_seconds: how many seconds to use a cached result from NTP server
+            (default 0, which respects NTP server's minimum).
         """
         self._pool = socketpool
         self._server = server
@@ -63,6 +66,7 @@ def __init__(
         self._packet = bytearray(PACKET_SIZE)
         self._tz_offset = int(tz_offset * 60 * 60)
         self._socket_timeout = socket_timeout
+        self._cache_seconds = cache_seconds
 
         # This is our estimated start time for the monotonic clock. We adjust it based on the ntp
         # responses.
@@ -74,7 +78,8 @@ def __init__(
     def datetime(self) -> time.struct_time:
         """Current time from NTP server. Accessing this property causes the NTP time request,
         unless there has already been a recent request. Raises OSError exception if no response
-        is received within socket_timeout seconds"""
+        is received within socket_timeout seconds, ArithmeticError for substantially incorrect
+        NTP results."""
         if time.monotonic_ns() > self.next_sync:
             if self._socket_address is None:
                 self._socket_address = self._pool.getaddrinfo(self._server, self._port)[
@@ -92,8 +97,11 @@ def datetime(self) -> time.struct_time:
                 # the packet.
                 destination = time.monotonic_ns()
             poll = struct.unpack_from("!B", self._packet, offset=2)[0]
-            self.next_sync = destination + (2**poll) * 1_000_000_000
+
+            cache_offset = max(2**poll, self._cache_seconds)
+            self.next_sync = destination + cache_offset * 1_000_000_000
             seconds = struct.unpack_from("!I", self._packet, offset=PACKET_SIZE - 8)[0]
+
             self._monotonic_start = (
                 seconds
                 + self._tz_offset

examples/ntp_connection_manager.py

@@ -33,7 +33,7 @@
 # get the socket pool from connection manager
 socket = adafruit_connection_manager.get_radio_socketpool(radio)
 
-# adjust tz_offset for locale...
-ntp = adafruit_ntp.NTP(socket, tz_offset=-5)
+# adjust tz_offset for locale, only ping NTP server every hour
+ntp = adafruit_ntp.NTP(socket, tz_offset=-5, cache_seconds=3600)
 
 print(ntp.datetime)

examples/ntp_cpython.py

@@ -9,7 +9,7 @@
 import adafruit_ntp
 
 # Don't use tz_offset kwarg with CPython because it will adjust automatically.
-ntp = adafruit_ntp.NTP(socket)
+ntp = adafruit_ntp.NTP(socket, cache_seconds=3600)
 
 while True:
     print(ntp.datetime)

examples/ntp_set_rtc.py

@@ -21,7 +21,7 @@
 wifi.radio.connect(secrets["ssid"], secrets["password"])
 
 pool = socketpool.SocketPool(wifi.radio)
-ntp = adafruit_ntp.NTP(pool, tz_offset=0)
+ntp = adafruit_ntp.NTP(pool, tz_offset=0, cache_seconds=3600)
 
 # NOTE: This changes the system time so make sure you aren't assuming that time
 # doesn't jump.

examples/ntp_simpletest.py

@@ -20,7 +20,7 @@
 wifi.radio.connect(secrets["ssid"], secrets["password"])
 
 pool = socketpool.SocketPool(wifi.radio)
-ntp = adafruit_ntp.NTP(pool, tz_offset=0)
+ntp = adafruit_ntp.NTP(pool, tz_offset=0, cache_seconds=3600)
 
 while True:
     print(ntp.datetime)