Using external UART modules to debug PSLab operations

Pocket Science Lab by FOSSASIA is a compact tool that can be used for circuit analytics and debugging. To make things more interesting, this device can be accessed via the user interface using an Android app or also a desktop app. Both these apps use the UART protocol (Universal Asynchronous Receiver-Transmitter) to transmit commands to the PSLab device from mobile phone or PC and receive commands vice versa. The peculiar thing about hardware is that the developer cannot simply log data just like developing and debugging a software program. He needs some kind of an external mechanism or a tool to visualize those data packets travelling through the wires.

Figure 1: UART Interface in PSLab

PSLab has a UART interface extracted out simply for this reason and also to connect external sensors that use the UART protocol. With this, a developer who is debugging any of the Android app or the desktop app can view the command and data packets transmitted between the device and the user end application.

This  requires some additional components. UART interface has two communication related pins: Rx(Receiver) and Tx(Transmitter). We will need to monitor both these pin signals for input and output data packets. It should be kept in mind that PSLab is using 3.3V signals. This voltage level is important to mention here because if someone uses 5V signals on these pins, it will damage the main IC. There are FTDI modules available in market. FTDI stands for Future Technology Devices International which is a company name and their main product is this USB transceiver chip. These chips play a major role in electronic industry due to product reliability and multiple voltage support. PSLab uses 3.3V USB Tx Rx pins and modules other than FTDI wouldn’t support it.

Figure 2: FTDI Module from SparkFun

The module shown in Fig.2 is a FTDI module which you can simply plug in to computer and have a serial monitor interface. There are cheaper versions in shopping websites like eBay or Alibaba and they will also work fine. Both Tx and Rx pins will require two of these modules and connectivity is as follows;

PSLab [Rx Pin] → FTDI Module 1 [Rx Pin]
PSLab [Tx Pin] → FTDI Module 2 [Rx Pin]

This might look strange because everywhere we see a UART module is connected Rx → Tx and Tx → Rx. Notice that our idea is to monitor data packets. Not communicate with PSLab device directly. We want to see if our mobile phone Android app is sending correct commands to PSLab device or not and if PSLab device is transmitting back the expected result or not. This method helped a lot when debugging resistance measurement application in PSLab Android app.

Monitoring these UART data packets can be done simply using a serial monitor. You can either download and install some already built serial monitors or you can simply write a python script as follows which does the trick.

import serial

ser = serial.Serial(
    port='/dev/ttyUSB1',
    baudrate=1000000000
)

ser.isOpen()
while 1 :
    data = ''
    while ser.inWaiting() > 0:
        data += ser.read(1)

    if data != '':
        print ">>" + data

Once you are getting commands and responses, it will look like debugging a software using console.

References:

  1. PySerial Library : http://pyserial.readthedocs.io/en/latest/shortintro.html
  2. UART Protocol : https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter
  3. FTDI : https://en.wikipedia.org/wiki/FTDI