I have learned the hard way that many R4 Wifi features are deeply flawed or outright worthless. I have wasted hundreds of hours trying to get things to work that were never going to. I can't get that time back, but I hope I can help others avoid that fate, so this is my catalog of all the ways the R4 fails.
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RTC
RTC Severe Time Drift and Inaccuracy The RTC suffers from extreme time drift that makes it unsuitable for any precision timing applications. Users report drift rates of 1 second per minute (equivalent to 24 minutes per day), with some experiencing 20 seconds drift in just 10 minutes. The internal clock can be off by 5+ minutes overnight, requiring daily manual corrections or constant NTP synchronization to maintain reasonable accuracy. This level of inaccuracy eliminates the RTC's utility for data logging, scheduling, or time-critical applications.
RTC Battery Backup Complete Failure Despite having dedicated VRTC and GND pins for external battery backup, the RTC consistently resets to either 00:00:00 or the initial programmed time whenever the USB is disconnected and reconnected, even with proper 3V battery connections. Users report trying both CR2032 coin cells and stable 3.3V power supplies with the same failure - the RTC never maintains time during power cycles, making the battery backup feature completely non-functional.
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RTC and Arduino Cloud
RTC/Arduino Cloud Corruption The onboard RTC works fine until you try to use it while sending data to Arduino Cloud, at which point the hour value gets reset to a different value while other time components remain correct. This makes time-sensitive cloud applications unreliable for precise scheduling or data logging.
NOTE: This has been significantly solved for per the post below. However, your RTC time then is now only as good as the last time your sketch successfully calls ArduinoCloud.update() minus whatever drift the onboard RTC experiences. For accurate time all the time the complete solution is to install an external, battery-operated clock, such as an Adafruit DS3231.
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I2C
I2C GPIO Expanders Completely Incompatible Extensive research reveals zero documented successful implementations of I2C GPIO expanders (PCF8574, MCP23017) with the Arduino R4 WiFi. Users report that Qwiic devices load sketches without problems but "do nothing," while the same components work fine with other Arduino boards. This eliminates a fundamental method for expanding pin availability, severely limiting project scalability.
I2C Reliability Issues I2C communication produces nonsensical data on R4 WiFi compared to perfect results on R3, with sensors like MPU6050 failing to work properly. Connections are unreliable, often requiring unplugging and replugging the Arduino after every upload for I2C devices to function. This makes the board unsuitable for projects requiring reliable sensor communication. Note: The disconnect/reconnect refers to unplugging the USB cable from the Arduino R4 WiFi board itself (not disconnecting I2C devices), requiring a full power cycle of the board after each sketch upload to restore I2C functionality.
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Arduino Cloud and OTA Updates
Arduino Cloud OTA Upload Failures for Large Sketches The Arduino R4 WiFi cannot reliably upload large sketches over-the-air (OTA) through Arduino Cloud. The OTA update process fails or times out when attempting to deploy sketches above a certain size threshold, forcing developers to rely on USB cable connections for updates. This severely limits remote update capabilities for deployed projects and eliminates a key advantage of WiFi-enabled boards for maintenance and feature updates in the field.
NOTE: You can still use the cloud, but you must upload your sketches with USB.
UART
TMC2209 UART Feedback Features Completely Non-Functional Extensive research reveals zero successful implementations of TMC2209 UART feedback features (StallGuard, CoolStep, sensorless homing) with the Arduino R4 WiFi. Combined with the board's documented UART interrupt failures where "exactly ONE interrupt occurs, then they stop working," the R4 WiFi appears fundamentally incompatible with advanced stepper driver features that rely on bidirectional UART communication. This eliminates critical modern stepper motor capabilities like sensorless homing and load detection.
WiFi
I have gotten it to work, but it is quite flaky.
WiFi Connectivity Instability The board struggles to connect to internet, and when it does connect, it frequently loses connection after brief periods, requiring manual reconnection interventions. Arduino Cloud connections fail to auto-reconnect when disconnected, requiring custom callback functions to restore connectivity.
EEPROM
I can't swear that EEPROM is deeply flawed because my observations may be confounded by known RTC issues, but if I never saw fire I saw plenty of smoke. EEPROM worked in the sense that it captured the bit of info it was supposed to capture, but not reliably as near as I could tell, so ultimately EEPROM did not work for me at all.
Rather than deleting AI text below, which I theoretically should because it is wrong, I'm leaving it for readers' context so when they see the thoughtful rebuttals below they will understand what others are responding to. The TL;DR version is that the R4 WiFi's Renesas R4AM1 does have native EEPROM memory, it is not emulated.
AI-IDENTIFIED BELOW
EEPROM Emulation Efficiency Problems The Arduino R4 WiFi doesn't have true EEPROM but uses flash memory emulation that requires erasing entire 1024-byte pages to write single bytes, causing rapid wear and potential premature failure. Expert recommendations advise against using the built-in EEPROM library for any data logging applications due to inefficiency and reliability concerns.
EEPROM Data Storage and Retrieval Reliability Issues The flash-emulated EEPROM system suffers from data corruption and unreliable storage/retrieval operations. Arduino's own technical documentation acknowledges these reliability concerns by recommending developers implement "marker bytes on EEPROM indicating valid information" and verification systems that check for valid data upon startup before trusting any stored values. This defensive programming approach suggests Arduino is aware that the emulated EEPROM frequently fails to store or retrieve data correctly. User reports confirm experiencing situations where data appears to write successfully but cannot be reliably retrieved, or where stored values become corrupted over time, making the EEPROM unsuitable for any critical data persistence needs.
Had posted AI-identified defect here that I had not personally encountered. Removed for fairness, but check any feature for viability for yourself before committing time to it.
Had posted AI-identified defect here that I had not personally encountered. Removed for fairness, but check any feature for viability for yourself before committing time to it.
Had posted AI-identified defect here that I had not personally encountered. Removed for fairness, but check any feature for viability for yourself before committing time to it.
Had posted AI-identified defect here that I had not personally encountered. Removed for fairness, but check any feature for viability for yourself before committing time to it.
Had posted AI-identified defect here that I had not personally encountered. Removed for fairness, but check any feature for viability for yourself before committing time to it.
Hi @catjampow. I'm sorry you haven't been satisfied with your UNO R4 WiFi board.
Have you personally verified these "AI-IDENTIFIED" claims? If not, I don't think it is fair to include them in a list of "Critical defects and limitations to know before you waste your time".
LLMs can be a useful tool, but they are prone to "hallucinations", so you must never take information from an AI as fact without first verifying it. It would be especially irresponsible to do so in this context.
It might well be that the AI's claims are based on random posts it found on the Internet. However, those of us who support such reports know very well that as often as not if we are able to perform a thorough investigation the reality of the situation is found to be something very different from what was initially presented.
For example
I have personally verified that the keyboard and mouse emulation of the UNO R4 WiFi work fine.
I'm not aware of any claims that the board has such a capability.
A USB host capability isn't something that is very commonly needed for use in Arduino projects, so I don't see that the lack of such a capability could be considered a "critical limitation".
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I have removed all instances of AI-identified issues.
What "extensive research"? Lots of claims here, with zero documentation.
Plenty of problems with the R4 WiFi have been documented and rather thoroughly discussed on this forum, but there is not a single reason for anyone to take this long, multi-part rant seriously.
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My extensive research over three working days to install an I2C expansion board and seven workdays trying unsuccessfully to get UART to work with a stepper motor.
If you choose dismiss this collection as a rant I would say that's understandable, but it's your loss. For everyone else, look for yourself before you invest in trying to get the R4 Wifi to perform any of these functions. And THAT is the point. Had I had the benefit of this "long, multi-part rant" early on, I could have done a bit of homework and saved myself a lot of needless effort.
Or, you could have posted the details of the problems you encountered on the forum, and had expert help in fixing them.
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I'm not going to argue with you any further other than to say that I reject your strawman argument that one has no credibility to comment on Arduino flaws unless you pass through the fires of seeking expert help via the forum.
I believe everything I have listed is intractable. That said, I'd LOVE to be wrong. For example, feel free to post a SINGLE working example of adding an I2C expansion board or using the TMC2209's advanced features. Don't bother with how Arduino Cloud trashes RTC; that's out of the horse's mouth.
Unfortunately the UNO R4 WiFi is not compatible with a lot of libraries, particularly older libraries that are not actively supported.
The quiic connector is on the Wire1 I2C bus, and is 3.3V and therefore incompatible with devices operating at 5V. A lot of code and libraries would require modification to adapt to this, and I'll agree it has caused much confusion.
There are similar issues with the serial port, because the Rx and Tx pins are Serial1, making it necessary to modify code written for the classic atmega328 based UNO.
One of the major failings of the R4 is that it has UNO in the name, leading to way too many people expecting it to be a drop-in replacement for a classic UNO.
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