How to set to a ROS system a fixed communication frequency?

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I have a ROS system with 2 nodes. The first node receives a string of data from a raspberry by using serial port and it uploads on in a topic. The second node subscribes to that topic, it gets the data and after some calculations it sends a string to a microcontroller using another serial port. The calculations include some equations that depend on time.

Let's say that I want to send the data to the microcontroller at a fixed frequency (66.67 Hz), but the calculations depend on data from all the system. How can I achieve that from the moment I run all the nodes the system is going to send the data to the microcontroller with this fixed frequency? Do I have to set the whole system under a common time frame? and if that is the case, how can I do it? I think that by setting only the rate to the second node to match that frequency will not solve my problem. After a small research I found that rospy.rostime might be helpful, but I didn't find any example on how to use it and what exactly can offer. I am new to ROS so I still try to find out how things work. I use ROS Kinetic, in Ubuntu 16.04 LTS and my nodes are in Python 2.7.12. Thank you in advance.

UPDATE:

The code running on my Raspberry Pi is:

#!/usr/bin/env python
import random
import decimal
import time
import serial

ser = serial.Serial(

    port = '/dev/ttyAMA0',
    baudrate = 115200,
    parity = serial.PARITY_NONE,
    stopbits = serial.STOPBITS_ONE,
    bytesize = serial.EIGHTBITS,
    timeout = 1
)

time.sleep(2)

counter = 0


while 1: # i while True:
    x = float(decimal.Decimal(random.randrange(0, 5010))/10)
    y = float(decimal.Decimal(random.randrange(0, 5010))/10)
    z = float(decimal.Decimal(random.randrange(0, 5010))/10)
    thita = float(decimal.Decimal(random.randrange(0, 5010))/10)
    u_x = float(decimal.Decimal(random.randrange(0, 5010))/10)
    u_y = float(decimal.Decimal(random.randrange(0, 5010))/10)
    string_buffer = str(x)+","+str(y)+","+str(z)+','+str(thita)+','+str(u_x)+','+str(u_y)+'/'+'\n'
    print string_buffer
    ser.write(string_buffer)
    time.sleep(1)
    counter += 1

ser.close()

and the code of the first node (receiving from Pi and publishing on topic) is:

#!/usr/bin/python

import rospy
import serial
import time
from std_msgs.msg import String

ser = serial.Serial('/dev/ttyS0', 115200)
print (ser.name)
time.sleep(2)

def camera_node():
    pub = rospy.Publisher('real_position', String, queue_size=10) # to$
    rospy.init_node('camera_node', anonymous=True)
    rate = rospy.Rate(1) #hz, mporei na thelei sygxronismo me ta ypolo$
    print("kokos")
    if ser.isOpen():
        print("in the loop")
        camera_str = ser.readline() #... auto p tha diavazei apo serial
        #print(camera_str)
        rospy.loginfo(camera_str)
        pub.publish(camera_str)
        rate.sleep()

if __name__ == '__main__':
    try:
        while not rospy.is_shutdown():
            camera_node()
        ser.close()
    except rospy.ROSInterruptException:
        pass