A few months ago, I got a requirement for a POC, to create a system to monitor an UPS. The inverter would provide its status, e.g. "Mains On", "Inverter On", "Battery Voltage", "Mains High", "Mains Low" etc. over a RS232 serial port. We had to read it and send it over the net to our website where the data would be stored and monitored. The readings were to be taken every 1 minute.
The Microcontroller
Reading UART serial data is not difficult for a very basic microcontroller. Even without a UART port, it is simple to read it using a program, as I did here. However, the requirement to post the data real-time to a website, called for network connectivity, and hence, a more powerful solution.
This is where the Raspberry Pi Pico comes in.
- Its a powerful microcontroller board, with many IOs including UART ports.
- It has a Pico W version, which comes with WiFi connectivity.
- Its cheap.( The base version is around Rs 350)
- It can be programmed with MicroPython, which is much easier to program in than C, and the Thonny IDE is very user friendly. Kudos to the MicroPython and Thonny devs !
While I thought I was done with the choice of microcontroller, a problem arose. There was no WIFI connection available onsite, only an Ethernet one. Now the Pico does not have a version with an Ethernet port. So i had to do some research again.
There were options where I could connect an additional Ethernet board to the Raspberry Pico, but then, I wanted it to be inbuilt, to keep it simple.
Fortunately, Wiznet provides Ethernet versions, modified versions of the Pico board, with an Ethernet port.
Now where would I get it in India ?
Digikey seems to be a great site for electronics, it has a huge catalogue and low prices, and volume discounts. However, i just wanted one item to start with, and the shipping charges were too high. Hence i opted for Hubtronics, which was costlier for the board, but overall cheaper due to the lower shipping charges.
Development
The Pico I had ordered is a development board, so it can be programmed easily. It is to be connected using a micro-usb cable to you computer. It has a switch which can be used to put it in 2 different modes :
- As a USB storage device. This mode will be used only once at the start, to copy the Micropython image to the Pico's internal storage. This is necessary to support the Micropython programming environment.
- As a USB device that interacts with the Thonny IDE to develop and test the Micropython programs.
Here's a tutorial on how to go about it.
Some Gotchas
Some problems that I encountered
- Voltage compatibility - The serial port signals were 0-5V, whereas the Pico's IO, like the Pi, is 0-3.3V. There are various solutions for this, from transistors to chips, but I did it with a resistive voltage divider, since the RS232 baud rate used was low.
- Male and Female pinout differences - This may not be immediately clear, but is obvious when you think of it. Male and Female pinouts are different, since Rx in one has to go to Tx in another and so on. So lookup the pinout as per the type of socket.
- UART parameters matching - When you initialize the Pico's UART, make sure that its parameters like baud-rate, parity, start-stop bits match that of the port you are interfacing with.
- Reads and timeouts - If you read multiple bytes at a time, and it times out, you may be get lesser data than what you asked for.
- Sleeps - Sleeps in between can make you miss data that is being sent in a stream.
- Micropython differences - Since Micropython is not the full python version, you will not have some commonly used classes, e.g. not all data structures are available.
