Started following some tutorial on att85 usi. Downloaded example code from make avr book.

Make AVR Examples/Chapter11_Driving-Servo-Motors/servoSundial/calibrateTime.py

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+
+
+import time
+import serial
+
+def readTime(serialPort):
+    '''Reads the time from the AVR over the serial port'''
+    serialPort.flushInput()
+    character = ""
+    while(not character == "\n"):     # loop until see end of line 
+        character = serialPort.read(1)
+    ## The time string looks something like '011:045:023\r\n'
+    timeString = serialPort.read(13) 
+    hms = timeString.split(":")
+    hms = [int(x) for x in hms] # make hour, minute, second numeric
+    return(hms)
+
+def setTime(serialPort, hours, minutes, seconds):
+    '''Sends the time over the serial port'''
+    serialPort.flushOutput()
+    serialPort.write("S")
+    time.sleep(0.1)             # delay while AVR sends
+    serialPort.write(str(hours) + "\r")
+    time.sleep(0.2)             # delay while AVR sends
+    serialPort.write(str(minutes) + "\r")
+    time.sleep(0.2)             # delay while AVR sends
+    serialPort.write(str(seconds) + "\r")
+    
+def setTimeNow(serialPort):
+    '''Sets the AVR clock to the current time'''
+    hours, minutes, seconds = time.localtime()[3:6]    
+    setTime(serialPort, hours, minutes, seconds)
+    return(time.time())
+
+def calculateTimeDelay(serialPort):
+    '''Gets AVR time and subtracts off actual (computer) time'''
+    avrHMS = readTime(serialPort)
+    hms = time.localtime()[3:6]
+    hmsDifference = [x - y for x,y in zip(avrHMS, hms)]
+    out = "AVR is fast by: {x[0]} hours, {x[1]} minutes, and {x[2]} seconds"
+    print out.format(x=hmsDifference)
+    return(hmsDifference)
+
+def calculateTimeDrift(serialPort, startTime):
+    '''Calculates the ratio to multiply OVERFLOWS_PER_SECOND 
+    given a start time and current error'''
+    h, m, s = calculateTimeDelay(serialPort)
+    driftSeconds = 60*60*h + 60*m + s
+    elapsed = time.time() - startTime
+    print "After {:.0f} seconds, ".format(elapsed)
+    return (driftSeconds / elapsed + 1)
+ 
+
+if __name__ == "__main__":
+    
+    ## Set time automatically, recording start time, 
+    ##  then periodically calculate multiplication factor
+    OVERFLOWS_PER_SECOND = 31250 # set this to equal the value in your code
+    
+    SLEEP_TIME = 10            
+    ratioLog = []
+
+    s = serial.Serial("/dev/ttyUSB0", 9600, timeout=5)
+    print "Setting time to current time...."
+    ratio = 0
+    while not ratio == 1:       # make sure starting time is right on
+        startTime = setTimeNow(s) 
+        ratio = calculateTimeDrift(s, startTime)
+
+    ## Note: you can either leave this running or 
+    ## you can re-run calculateTimeDrift() at any time in the future, 
+    ## as long as you don't overwrite the original startTime 
+    while(True):
+        ratio = calculateTimeDrift(s, startTime)
+        ratioLog.append([time.time()-startTime, ratio])
+        newOverflow = int(OVERFLOWS_PER_SECOND * ratio)
+        print "OVERFLOWS_PER_SECOND should be {}\n\n".format(newOverflow)
+        time.sleep(SLEEP_TIME)
+        
+    ## As you leave this routine running, you should see it bounce
+    ##  around a lot in the beginning and then settle down after
+    ##  running a few hours.  Ironically, it'll converge to a good
+    ##  number faster if it's initially very out of sync.  (If it
+    ##  drifts faster, you can figure out the drift rate sooner.)
+    ## Leave it running for 24 hours and you'll get one-second-per-day 
+    ##  accuracy.