Big latency in bluetooth communication - c

Question

I have tried to write wireless servo control using two arduino nano v3 boards and two bluetooth 4.0 modules. First code is transmitter. It's very simple. It reads PPM signals and transform to separates PWM values for each channel. I use hardware serial port.
#include <PPMReader.h>
#include <InterruptHandler.h>
int ppmInputPin = 3;
int channelAmount = 2;
PPMReader ppm(ppmInputPin, channelAmount);
void setup()
{
Serial.begin(9600);
Serial.write("AT\r\n");
delay(10);
Serial.write("AT\r\n");
Serial.write("AT+INQ\r\n");
delay(5000);
Serial.write("AT+CONN1\r\n");
}
void loop()
{
unsigned long value1 = ppm.latestValidChannelValue(1, 0);
Serial.println(value1);
}
Receiver is simple too. It reads values from bluetooth and parse into integer value and sends to servo by 7th pin. Again I have used hardware serial port.
#include <Servo.h>
int PWM_OUTPUT = 7;
Servo servo;
void setup() {
servo.attach(PWM_OUTPUT);
Serial.begin(9600);
}
void loop() {
int pwmValue = Serial.parseInt();
if (Serial.available()) {
if(pwmValue > 900 && pwmValue < 2001) {
servo.writeMicroseconds(pwmValue);
}
}
}
All it works. But it has delay around 2-3 seconds. Can be problem in "spamming" serial port?

Answer 1

The first thing you need to ask yourself when implementing a device-to-device communication is how fast should I be sending? and if I send at that rate: is the receiver going to be able to keep pace (reading, doing processing or whatever it needs to do and answer back)?
This is obviously not about the baud rate but about what your loops are doing. You are using two different libraries: PPMReader and Servo. Now, pay attention to what each device is doing in their respective loops:
//Sending
void loop() {
unsigned long value1 = ppm.latestValidChannelValue(1, 0);
Serial.println(value1);
}
//Receiving
void loop() {
int pwmValue = Serial.parseInt();
if(pwmValue > 900 && pwmValue < 2001) {
servo.writeMicroseconds(pwmValue);
}
}
I don't really know how long it takes to execute each line of code (take a look here for some comments on that) but you cannot seriously expect both loops to magically synchronize themselves. Considering they are doing very different things (leaving out the serial part) dealing with different hardware, I would expect one of them to take significantly longer than the other. Think about what happens if that's the case.
As I said, I have no idea how long it takes to call ppm.latestValidChannelValue(1, 0) but for the sake of my argument let's say it takes 0.1 milliseconds. To have an estimate of the time it takes to complete one iteration around the loop you need to add the time it takes to print one (or two) bytes to the port with Serial.println(value1) but that's easier, maybe around 20-100 microseconds is a good ballpark figure. With these estimates, you end up reading 5000 times per second. If you are not happy or you don't trust my estimates I would suggest you do your own tests with a counter or a timer. If you do the same exercise for the other side of the link and let's say you get it's twice as fast, it runs 10000 times per second, how do you think it would happen with the communication? Yes, that's right: it will get clogged and run at snail pace.
Here you should carefully consider if you really need that many readings (you did not elaborate on what you're actually doing so I have no idea, but I lean on thinking you don't). If you don't, just add a delay on the sender's side to slow it down to a reasonable (maybe 10-20 iterations per second) speed.
There are other things to improve on your code: you should check you have received data in the buffer before reading it (not after). And you need to be careful with Serial.parseInt(), which sometimes leads to unexpected results but this answer is already too long and I don't want to extend it even more.

Answer 2

I found problem. It was in serial port spamming. I have added check if current value is not equal with previous value and it have started work and next small issue was in receiver. I read value before it was available.
#include <PPMReader.h>
#include <InterruptHandler.h>
int ppmInputPin = 3;
int channelAmount = 2;
PPMReader ppm(ppmInputPin, channelAmount);
volatile unsigned long previousValue1 = 0;
void setup()
{
Serial.begin(9600);
Serial.write("AT\r\n");
delay(10);
Serial.write("AT\r\n");
Serial.write("AT+INQ\r\n");
delay(5000);
Serial.write("AT+CONN1\r\n");
Serial.println("Transmitter started");
}
void loop()
{
unsigned long value1 = ppm.latestValidChannelValue(1, 0);
if(previousValue1 != value1) {
previousValue1 = value1;
Serial.println(value1);
}
}