Two Arduino UNO R4 Boards: Minima and WiFi

Announced a few months ago, the Arduino UNO R4 Minima and the Arduino UNO R4 WiFi have now been released officially. So, now that they really exist, let’s have a closer look at them.

The Renesas Family

A few months ago, Arduino released the Portenta C33 board, which has an ARM Cortex-M33 microcontroller from Renesas: the RA6M5. The two new boards also feature a device from Renesas, the RA4M1. This 32-bit ARM Cortex-M4 runs at 48 MHz and has 32 KB of RAM and 256 KB of flash memory. It seems that a family of Renesas boards is starting to develop.

Interesting to note is that the RA4M1 can work with a power supply of up to 5 V whereas most other ARM-type microcontrollers require 3.3 V. This makes the MCU a suitable candidate for enhancing the 5-V, 8-bit AVR-based family of which the Arduino UNO R3 is a famous member.
 

More Peripherals on the UNO R4

Replacing an old 8-bit, 28-pin controller by a modern 32-bit 64-pin device has, as you may expect, an impact on the complexity of the product. However, this touches mainly the software part as indicated by the MCU’s 1400+-page-long datasheet (<300 pages for the ATmega328). The UNO R4 Minima board is of similar design complexity as the R3. The UNO R4 WiFi is a denser board as it uses the empty board space of the Minima for an 8 by 12 LED matrix and an ESP32-S3-MINI-1 module.

The datasheet is that long because the Renesas MCU features many more peripherals than the Microchip ATmega328. Not all of these are supported by the R4s because not all the pins of the MCU are accessible (it has a 64-pin package. A 100-pin version exists too, maybe an idea for a new Mega board?), but a few nice ones are. These include a CAN bus and USB 2.0 Full-Speed (host or device).

A USB-C connector replaces the USB-B connector.

More Comms?

The RA4M1 integrates two SPI ports, two I²C ports plus four serial communication interfaces (SCIs). An SCI can be either a UART, an I²C master or a simple SPI port (i.e. up to six I²C or SPI ports), and even a smart card interface. According to Arduino, these ports are available, at least to some extent.^* However, looking at the schematic of the Minima, only one I²C port appears to be connected. Pins A4, A5, D4 and D5 expose a second SPI port even though the board specifications mention only one. A second serial port (Serial1) is available too, but it shares its pins with the SPI port.
   

The UNO R4 Has Better Analog

The UNO R4 features a 12-bit digital-to-analog converter (DAC) for producing real analog signals instead of PWM-based surrogates. There are also an opamp and a comparator together with an internal 8-bit DAC and the analog-to-digital converter ADC is 14-bit wide instead of 10 bits for the UNO R3. The analogWave library was added to make using the DAC easy. Generating a sine, saw or square wave is as easy as calling a library function. Of course, you can do much more with it. Therefore, on the analog side, the UNO R4 has much more to offer than the UNO R3.

Added Software Complexity

Regarding the Arduino IDE, switching to a new, hitherto unsupported processor family also implies adding software support. As we have learned over the years in our computer-controlled world, new software tends to introduce issues, and so it will probably take some time before the UNO R4 experience will become as smooth as that of the UNO R3.
 

Luckily, the UNO R4 Minima makes resolving issues somewhat easier as it has an SWD interface, which allows for serious (serial) debugging. The UNO R4 WiFi, takes this even a step further as its ESP32-S3 module can act as an on-board CMSIS-DAP debugger.

Wi-Fi Modem

Now that we mentioned the Arduino UNO R4 WiFi, let’s see in what way it differs from the UNO R4 Minima. First of all, there is, of course, the Wi-Fi & Bluetooth LE module, an ESP32-S3 from Espressif. It communicates with the MCU over a serial port (Serial2) in AT-command mode. The Wi-Fi module’s other pins are exposed as tiny solder pads. Reprogramming the module is possible as the required pins for this are accessible on a 2 x 3 header (and on the bottom side of the PCB). The new Arduino library WiFiS3 provides high-level software support for the module.

LED Matrix

A 96-pixel (8 × 12) red LED matrix enables users to plot data, create animations and provide more complex and sophisticated feedback in projects. A new library provides functions to show animations on it^**. A web tool for designing animations has been announced too.
 

The matrix uses Charlieplexing to connect the 96 LEDs to only 11 GPIO ports (D28 to D38 in Arduino notation). This means that only a few LEDs can be active at any one time as pixels consist of two LEDs connected in antiparallel, and pixels share ports. However, because the human eye is slow, fast time multiplexing allows tricking the brain into seeing complete images.

A Second I²C Port

The UNO R4 WiFi features two I²C ports (Wire & Wire1). A Qwiic-compatible (SparkFun standard) I²C connector provides access to the second port. They may be annoyingly tiny, but as the Qwiic standard imposes a 3.3-V supply voltage, the board comes with a level shifter. This means that the board can do both 5-V and 3.3-V I²C communications.

Power Supply

When not powered from the USB port, the power supply on the Arduino UNO R3 is a basic linear voltage regulator. On the UNO R4 boards, this has been replaced by a switching regulator. It allows a much wider input voltage range from 6 V up to 24 V. The regulator, also a Renesas product, can deliver up to 1.2 A with an efficiency of around 90%.
Note that the UNO R4 WiFi also has a connector to power the real-time clock (RTC).
 

Conclusion

The two Arduino UNO R4 boards, Minima and WiFi, look like credible successors to the UNO R3. They have the same outline, extension connectors, and full 5 V I/O. The UNO R4 WiFi is a bit like a UNO R4 Minima with an extension shield built in. With its Wi-Fi module, LED matrix and Qwiic connector, it is easy to build IoT applications with.

The UNO R4 is way more powerful than the UNO R3, with more of everything, from memory to peripherals and operating speed. I had to install a special Renesas boards package in the IDE first before I could do any programming, but when you read these lines, that should no longer be necessary (according to Arduino). Therefore, to most users, the UNO R4 will be a drop-in replacement for the UNO R3.

The UNO R4 is a Great Step Forward

Finally, very interesting is the debugging connector on the Minima and the CMSIS-DAP debug capabilities of the ESP32-S3 module on the WiFi, a feature many developers have been waiting for since about the beginning of Arduino. This takes the Arduino UNO finally into the pro-development arena.Therefore, the Arduino UNO R4 can be considered a powerful step forward.
 

* Please keep in mind that this article is based on version 0.8.7-ea (Early Access) of the Arduino Renesas Boards package, so things may and probably will have evolved since.

** Before the LED matrix library became available, I had already written a little program to display a scrolling message on the UNO R4 WiFi. You can download it from GitHub.

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Livestream: Arduino + Renesas Talk UNO R4!

Learn about the Arduino UNO R4 from Arduino and Renesas engineers. Join Elektor live on YouTube on June 28 (4 PM Berlin). Watch live for:
 

• Stuart Cording gets the full lowdown about the UNO R4 from Alessandro Ranellucci (VP/Head of Arduino Makers) and Robert Nolf (System Architect with Renesas)
• Special guests from the maker community will offer their takes on this major update.
• Some special giveaways!