Python Arduino Serial Communication Speed

Question

Background : Im controlling serial manipulator using Arduino Uno with Servo Shield. Servo work at 20 ms, so i would like to send command atleast every 2.5ms. Command sent from PC.

I send 8 bytes of data from my python code on PC at 250k Baudrate. If i send it every 25 ms, my robot could respond and everything works fine. But if i send every 2.5 ms, my system wont work.

Here my arduino code,

#include <Wire.h>
#include <Adafruit_PWMServoDriver.h>
Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();
int x,temp;
int sudutT0(long int p)
{
 int u = 3600+((405*p)/180);
 return u;
}
void setup() {
 Serial.begin(250000);
 Serial.println("Joint 2 Test!");
 pwm.begin();
 pwm.setPWMFreq(50);
 Wire.setClock(1000);
 for (uint8_t pin=0; pin<16; pin++) 
 {
 pwm.setPWM(pin, sudutT0(90),4095);
 }
 pwm.setPWM(3, sudutT0(150), 4095);
}
void loop() {
 if(Serial.available() == 8){
 if(Serial.read() == 0xF5)
 {
 //Joint 1
 temp = Serial.read();
 x = sudutT0(temp);
 pwm.setPWM(0,x,4095);
 // Joint 2 Servo
 temp = Serial.read();
 x = sudutT0(temp);
 pwm.setPWM(1,x,4095);
 x = sudutT0(180-temp); //810 - theta
 pwm.setPWM(2,x,4095);
 //Joint 3 - 6, Gripper
 for(uint8_t u = 3; u<8; u++)
 {
 temp = Serial.read();
 x = sudutT0(temp);
 pwm.setPWM(u,x,4095);
 }
 //delay(100);
 }
 }
}

Is there anything that i miss, that make my system could not respond if i send signal every 2.5 ms?

• Chad G: Is there a reason you need to update it 400 times a second? All of the code takes time. It takes ~0.2 ms just to send the 8bytes at 250k, then you have to process the data. I would suspect that all that is taking longer then 2.5 ms and so by the time you read again, you have more then 8 bytes in the buffer, and eventually it overflows. I would try decreasing from 25ms to 24 and re-test, then reducing again. you may be able to find the lower limit of what will work reliably. This is assuming there is a need for it to update faster then 40 times a second, if not then just stay at 25ms.
• Albert H M: @ChadG Actually i wanna apply control theory about multirate sampling time, and i just realize i made a mistake.. i didnt need to make it 2.5ms now. And thank you for explaining why my robot couldn't respond.. could u put ur answer at answer section, because u explain why my robot not responding. Thx

Answer 1

All of the code takes time. It takes ~0.2 ms just to send the 8bytes at 250k, then you have to process the data. I would suspect that all that is taking longer then 2.5 ms and so by the time you read again, you have more then 8 bytes in the buffer, and eventually it overflows. I would try decreasing from 25ms to 24 and re-test, then reducing again. you may be able to find the lower limit of what will work reliably. This is assuming there is a need for it to update faster then 40 times a second, if not then just stay at 25ms