Finding accelerometer update frequency

[phishboh]

I tried to find the accelerometer update frequency on my N95 using the following code:

import appuifw, sys, e32
import time, math
import axyz    # accelerometer

class Accelerometer:

    p_text_colour = (200, 20, 20)
    p_buffer_time = 3.0    # [s]
    p_acc_frequency = 10.0 # [Hz]
  
    def __init__(self):
        self.acc_x = int(self.p_buffer_time * self.p_acc_frequency) * [0.0]
        self.acc_y = int(self.p_buffer_time * self.p_acc_frequency) * [0.0]
        self.acc_z = int(self.p_buffer_time * self.p_acc_frequency) * [0.0]
        self.acc_time = int(self.p_buffer_time * self.p_acc_frequency) * [0.0]
        self.acc_time2 = int(self.p_buffer_time * self.p_acc_frequency) * [0.0]
        
        self.exitflag = 0
        self.canvas = appuifw.Canvas(event_callback=self.event)
        appuifw.app.body = self.canvas
   
    def frame_rate(self, timestamps):
        try:
            frequency = len(timestamps) / (timestamps[-1] - timestamps[0])
        except:
            frequency = -1
        return frequency
         
    def update(self):
        appuifw.app.body.clear()
        appuifw.app.body.text((10, 10), u"ACC: %3.1f Hz (%4.1f, %4.1f, %4.1f)" % (self.frame_rate(self.acc_time), self.acc_x[-1], self.acc_y[-1], self.acc_z[-1]), self.p_text_colour)
        appuifw.app.body.text((10, 20), u"ACC: %3.1f Hz" % self.frame_rate(self.acc_time2), self.p_text_colour)
        
    def event(self, ev):
        pass
  
    def set_exit(self):
        self.exitflag = True
    
    def start_data(self):
        axyz.connect(self.accelerometer_data)
                
    def stop_data(self):
        axyz.disconnect()
        
    def run(self):
        appuifw.app.exit_key_handler = self.set_exit
        self.start_data()
        while not self.exitflag:
            self.update()
            e32.ao_sleep(0.00000001)
        self.stop_data()
        
    def accelerometer_data(self, x, y, z):
        timestamp = time.time() + math.modf(time.clock())[0]
        self.acc_time2 = self.acc_time2[1:]
        self.acc_time2.append(timestamp)        
        if (timestamp - self.acc_time[-1]) >= 1 / self.p_acc_frequency:
            self.acc_time = self.acc_time[1:]
            self.acc_time.append(timestamp)
            self.acc_x = self.acc_x[1:]
            self.acc_x.append(x)
            self.acc_y = self.acc_y[1:]
            self.acc_y.append(y) 
            self.acc_z = self.acc_y[1:]
            self.acc_z.append(y)                 
    
app = Accelerometer()
app.run()

I would also like to be able to set the update frequency using p_acc_frequency.

Some questions:
1) Why are the frequency measurements so unstable and why does (timestamp - self.acc_time[-1]) sometimes return negative values?
2) How can I improve the frequency measurements, so that I can get the accelerometer data in the buffer at a specified rate, e.g., 10 Hz?
3) Why is the e32.ao_sleep command necessary?
4) Any other comments/improvements on my very first accelerometer code are welcome.

[phishboh]

To get the timestamp I used:

time.time() + math.modf(time.clock())[0]

Is there a better way?