Category: Cross-Talk

• Why RS-485?

  Hello people!

  It’s been a while since my last post. I actually did a little summer activities (hiking, gardening, rearranging pantry, making soap etc). BUT I also managed to design the circuit schematic for my serial communication project.

  The schematic as shown in Figure 1 is very drafty. I was using Altium Designer for it, but since I am not making PCB at this moment, I thought this should be doable as my breadboarding reference. Again, this schematic is awful because you can’t really see the flow from the original to the isolation section. I will fix it later!

  

  Breadboarding part I: Look at those loops!

  

  Then I realized that those loops could introduce unwanted inductance, so I revised.

  

  As you can see, they look so much better! FLAT. You’d want them flat like that. It’s a tedious work, but I shall get used to it.

  For multigiga serial links (with increasing bitrates), it would suffer from signal distortions. While SerDes receiver designed to compensate those distortions, by creating a bigger eye opening (see: eye diagram) for reliable sampling, the bit rates also rising faster than Rx, PCB, and package HDI. This results in decreasing reachable total channel. RS-485 is a type of repeater that could restore the signals, hence RS-485 extending the channel reach.

  

  RS-485 use differential signaling over twisted pair, which is beneficial. We know from circuit theory, that differential signaling technique reduces electromagnetic interference (EMI) that could cause cross-talk. In addition, RS-485 also use 3-state logic (1, 0 and high Z) that allows individual transmitter to be deactivated.

  The schematic and circuit board above (see Figures 1,2,3), uses RS-485 as a repeater and it connected it to digital isolator to ensure the power-supply voltage to the MCU is compatible and DC-DC converter to rectify the whole process back to the DC.

  If you notice, the circuit diagram for DC-DC converter contains Inductor(L) and Capacitor (C). The LC circuit is a simple low pass filter to filter out unwanted signals that may present above the wanted pass band.

  So in theory, once we connect all of these out to the UTP cable, 50m cable should not signal faster than 2Mbit/s. We’ll see!

  Will keep you updated.

  Source: https://www.intel.com/content/www/us/en/io/serial-bus-white-paper.html

  August 25, 2019

  Electrical Engineering, repeater, RS485, Signal Integrity

  ---

• Test of Arduino’s USART

  Heya folks,

  Happy weekend to all of you! We had heavy rain on Saturday morning, and it was nice and good for my plants.

  We did a simple linear test with Arduino by using the Tx pin (USART) and to test the highest baud rate it can handle before it introduce some weird stuff on oscilloscope.

  //Again, pin#4 being LOW means for HIGH Z
  //the baud rate in this code is the highest rate that we tested and it had some nasty wave on oscilloscope
  void setup() {
    Serial.begin(2500000);
    pinMode(4, INPUT);
    digitalWrite(4, LOW); 
  }
  void loop() {
     Serial.write(0xAA);
  }

  Baud Rate(Bps)	Period ( μs)	Frequency(Hz)
  9600	~104.2	9596
  19200	~52.07	19204
  38400	~26.04	38402
  76800	~13.02	76804
  153600	~6.512	153562
  230400	~4.509	221778
  460800	~2.004	499001
  921600	~1.002	998003
  1843200	~0.5012	1995000
  2000000	~0.5011	1995609
  2500000	~0.5014	1995609

  Based on data above, we noticed that although it maintain the linear behavior, the higher the baud rate (the corresponds frequency value should be very close to the baud rate), the higher the deviation of the frequency rate.

  Check out the screenshots from two different baud rate:

  

  

  Just by eyeballing those two screenshots, we know that at period 250nanoseconds, the square wave behave abnormally (it has some negative peak, which we don’t want).

  We could say that it is feasible to transmit 1Mbs, BUT definitely less than 2.5Mbs!

  Tomorrow we will use two Arduino to test both Tx and Rx modules. That’s all for now, enjoy the rest of your weekend.

  Also, check out Thom Yorke’s new album:

  Cheers,

  August 11, 2019

  Arduino, Digital, Electrical Engineering, Oscilloscope, Signals, Testing, Transmitting

  ---

• Clock Slips

  Heya folks,

  Few days ago I received my some of the tools for my electronic projects (such as: oscilloscope, Arduino and Multimeter). We (William and I), configured an Arduino to test the oscilloscope by creating a simple square waves. Based on today’s experiment, we accomplished/discovered two interesting things:

  1. Near perfect square wave with no phase shift (117kHz frequency).

  We tested to see the frequency for 50% (no scale) by using analogWrite() function (reference).

  For this configuration, we noticed no phase shift on the analog side of the equation. However, after we tested with digitalWrite() (see reference), we noticed 1/4 of phase shift on our square wave (will update with image later).

  To solve this clock slips/phase shift problem, we did a little research and concluded that one of the reasons of the shift problem is due to interruption on the main (Arduino) code. Hence, we modified the code by adding noInterrupts() (see: reference).

  //The commented code were used to test elimination of phase 
  // shift before using the noInterrupts() function
  //unsigned int usdelay =1; 
  void setup() {
    noInterrupts();
    pinMode(3,OUTPUT);
    pinMode(4,OUTPUT);
    digitalWrite(3, LOW);
    digitalWrite(4, LOW);
  }
  
  void loop() {
    // put your main code here, to run repeatedly:
    digitalWrite(3, HIGH);
    digitalWrite(4, HIGH);
    //delayMicroseconds(usdelay);  
    digitalWrite(3, LOW);
    digitalWrite(4, LOW);
    //delayMicroseconds(usdelay);
  }
  

  

  For amplitude 5V, we managed to produce desired frequency: 117kHz (1 pin as INPUT) or 60.9kHz (for two pins as INPUT) without phase shift with the noInterrupts() solution.

  2. Cross-talk problem using serial connection

  We tested the serial connection with 9600bps and using 0xAA (Binary: 1010 1010 <– you can also imagine this as LOW, HIGH, LOW, HIGH,…, HIGH) pattern, and somehow our configuration introduced a cross-talk problem from the pin#4 we set-up as input (!!!).

  void setup() {
    Serial.begin(9600);
    pinMode(4, INPUT);
    digitalWrite(4, LOW);// pin#4 LOW means High Z
    }
  
  void loop() {
    Serial.write(0xAA);
  }

  

  As we can see from Figure 2, channel 2 (blue) is the square wave from pin#4 and channel 1 (yellow) is the square wave from the serial connection (tx) from the Arduino. We noted that the amplitude of the pin#4 is very low (148mV) compared to the amplitude of channel 1 (5.12V).

  That’s all for today. Will test 2 Arduinos later.

  Peace, Love, Mango

  August 7, 2019

  Arduino, electrical engineer, Oscilloscope, Signal Integrity, square wave

  ---