How to send float values between Arduino over UART?

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#31 18247101 29 Oct 2019 21:19

krisRaba krisRaba

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Post #31
18247101 29 Oct 2019 21:19

Generally now there is a meatloaf number two, because the standard UART is not a multidrop network, so in fact you should only directly connect 2 devices via UART or use a transceiver e.g. RS485, which makes it possible to connect more receivers to the communication lines. Or use two UARTs in one esp (if there is such an option there, because I don't know how many are derived), then you know with whom and what you are talking about, well and you have a point-to-point connection, not a multidrop. If you were using something like RS485, though, you could enter the address of the controllers and then you send the ID and the variable, and then you're able to recognise what the data is and from whom. Because just the bytes of the variable in the buffer won't tell you, and you can put those bytes into anything and the "system" won't stammer...
And what topology do you have? The 3 esp are at what distance from each other? Because maybe you only need one that handles all the slaves over I2C and the whole thing is overkill? Because you probably have enough resources in a single esp.

Added after 2 [minutes]:

PS. if you have some longer distances, then pulling a bare UART out into the world is not a very good idea. And if short, you don't need 3 esp .
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#32 18247123 29 Oct 2019 21:28

Anonymous Anonymous

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Post #32
18247123 29 Oct 2019 21:28

The problem is that Serial1 on the ESP8266 cannot be used to read data, but only to send it .
http://arduino.esp8266.com/Arduino/versions/2.3.0/doc/reference.html#serial

Added after 4 [minutes]: .

You can use the so-called software-serial in the ESP8266: https://github.com/plerup/espsoftwareserial
Then you can select different pins in the esp for the second serial port.
At what distance from each other are these esp?
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#33 18247178 29 Oct 2019 21:51

omnixcrs omnixcrs

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Post #33
18247178 29 Oct 2019 21:51

They are not at any distance. They are placed on a single board except that it is a multiprocessor circuit because of the implementation of different sentences in different modes.

Added after 4 [minutes]: .

I am not using any serial division at the moment.
#34 18247196 29 Oct 2019 21:54

Anonymous Anonymous

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Post #34
18247196 29 Oct 2019 21:54

Just connect them via the I2C bus with the proviso that only one of them can be the master. You already have information on this in the post #21 .

Obviously this single master will need to support other I2C slaves, namely the expander and the display.
#35 18247389 29 Oct 2019 23:15

krisRaba krisRaba

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Post #35
18247389 29 Oct 2019 23:15

That is, you could probably do it on one, you just need to organise the code better and split it functionally similar to how you now split it into several esp From what you write, you are operating on expanders anyway, the display you have with the controller etc.
Such a pseudo-multicore, it's more for complex tasks where you can't fit in one, or for redundancy .
Take a look at this to see if you're not overcomplicating .
And if you want to have functionally split blocks, some multitasking takes care of that and one thread doesn't have to worry about what the other is doing independently, and if you need to, you push the variable between threads, not between microcontrollers .
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#36 18265279 08 Nov 2019 10:45

omnixcrs omnixcrs

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Post #36
18265279 08 Nov 2019 10:45

Hello gentlemen,
I have followed your advice and simplified the circuit. I have one esp-12f handling one expander for buttons and lcd 2x16 over i2c and a NodeMcu V3 handling an expander for outputs. The connection between them is just 5v 0v tx and rx - nothing more. I2c are separate. Everything seems to work, but unfortunately I still have problems with uart communication - one moment it is good, the next just 0 etc. My device can work in two modes AUTO and MANUAL. In case of AUTO mode nodemuc sends variables to esp from time to time and here rather everything works, but in case of MANUAL mode communication must be in both directions and here it is worse. I have included below simplified communication codes, maybe you can find something:

Code on the nodemc:
Code: C / C++
Log in, to see the code
.

Code on esp:

Code: C / C++
Log in, to see the code
#37 18265591 08 Nov 2019 14:09

Anonymous Anonymous

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Post #37
18265591 08 Nov 2019 14:09

Try using the Serial.availableForWrite() function:
https://www.arduino.cc/reference/tr/language/...tions/communication/serial/availableforwrite/
Below is an example of modified UART_send() function code for the ESP-12:
Code: C / C++
Log in, to see the code
.
There may be a side effect of not sending data despite the condition (mode_button == 1) being met , because there will not be enough space in the output buffer. You should handle such a situation by, for example, restarting the timer that calls UART_send().
#38 18266390 08 Nov 2019 20:48

omnixcrs omnixcrs

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Post #38
18266390 08 Nov 2019 20:48

Ok I'll give it a try. And tell me how does the transmission over UART work exactly. E.g. if I send 48 bytes to esp this is how much space I take up in its buffer and now when I read them with Serial.readBytes they are removed from the buffer or do they still take up these 48 bytes?
#39 18266399 08 Nov 2019 21:01

Anonymous Anonymous

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Post #39
18266399 08 Nov 2019 21:01

They will be deleted if they have been read.

Added after 10 [minutes]: .

Serial.write() returns information about the number of "sent" bytes. You can also check this additionally.
If the output buffer is overflowing, then Serial.write() blocks the program from running, hence why I suggested using the Serial.availableForWrite() function to check exactly whether a packet of data of a certain size can be sent.
#40 18266544 08 Nov 2019 21:55

omnixcrs omnixcrs

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Post #40
18266544 08 Nov 2019 21:55

Well you're right, sometimes the programme would hang up on me. I now know why. And why does the send buffer overflow occur and how to avoid it?
#41 18266553 08 Nov 2019 22:00

Anonymous Anonymous

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Post #41
18266553 08 Nov 2019 22:00

omnixcrs wrote:
And why does the send buffer overflow occur and how to avoid it ?
.
As I wrote earlier, the buffer has a limited size. You should not send more than is possible at any given time i.e. has not been received by the other party. The data rate over the UART is also limited.
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#42 18266571 08 Nov 2019 22:14

omnixcrs omnixcrs

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Post #42
18266571 08 Nov 2019 22:14

Well ok, but if I send every 35 seconds and receive every 20 then everything will always be received, yes?
#43 18266639 08 Nov 2019 22:49

Anonymous Anonymous

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Post #43
18266639 08 Nov 2019 22:49

omnixcrs wrote:
No ok, but if I send every 35s and receive every 20 then everything will always be received, yes?
.
omnixcrs wrote:
but in the case of MANUAL mode the communication must be two-way and here it is already worse.
.
Serial-Serial communication takes place in half-duplex mode. This means that at any given time the UART can either send data or receive data, but not simultaneously. As the UART cannot send data, it will be held in the output buffer until sending is possible.
#44 18266656 08 Nov 2019 22:57

krisRaba krisRaba

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Post #44
18266656 08 Nov 2019 22:57

Why is it so strange there? Is there some RS transceiver? Or such stupidly written libraries?
After all, normally the UART laughs in full-duplex....
#45 18266679 08 Nov 2019 23:17

Anonymous Anonymous

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Post #45
18266679 08 Nov 2019 23:17

krisRaba wrote:
Why is it so strange there? Is there some kind of RS transceiver? Or such a stupidly written library?
.
Instead of Serial.begin(512000) you could use e.g. Serial.begin(512000, SERIAL_8N1, SERIAL_FULL) .
On both sides, of course. This option with SERIAL_FULL appeared in the ESP8266 and ESP32. It is not present in the standard Arduino HAL.
#46 18266684 08 Nov 2019 23:19

krisRaba krisRaba

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Post #46
18266684 08 Nov 2019 23:19

So, however, stupidly written libraries
#47 18266700 08 Nov 2019 23:27

Anonymous Anonymous

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Post #47
18266700 08 Nov 2019 23:27

omnixcrs wrote:
No ok, but if I send every 35 s and receive every 20 then everything will always be received, yes?
.
There may also be some interference or dropouts. At what distance are these ESPs from each other?
#48 18266718 08 Nov 2019 23:34

krisRaba krisRaba

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Post #48
18266718 08 Nov 2019 23:34

By the way, why make the programme so complicated and make your life more difficult? In normal MCUs the reading is done when something arrives, not artificially waiting for unknown amounts of time.
In between these 20s intervals you put everything to sleep so that it can't check whether data has already arrived?
#49 18266735 08 Nov 2019 23:43

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Post #49
18266735 08 Nov 2019 23:43

krisRaba wrote:
In normal MCUs the reading is done when something comes in, not artificially waiting for unknown amounts of time.
.
In this "normal" MCU you can also using handlers for interrupts from the UART.
Here is a simple example:
https://github.com/nkolban/Sample-ESP8266-App/tree/master/Sample%20FreeRTOS%20App

Added after 32 [seconds]:

krisRaba wrote:
Between these 20s intervals you put everything to sleep that it can't check for itself if data has already come in?
.
It doesn't put itself to sleep. It can do other things during that time.
#50 18267641 09 Nov 2019 16:56

krisRaba krisRaba

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Post #50
18267641 09 Nov 2019 16:56

Ok, so it can be done right, just a matter of reaching for the right sources. I already thought they made it all so clunky and lame .
#51 18268198 09 Nov 2019 21:25

omnixcrs omnixcrs

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Post #51
18268198 09 Nov 2019 21:25

no distance, they are next to each other on the board
#52 18268380 09 Nov 2019 23:15

krisRaba krisRaba

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Post #52
18268380 09 Nov 2019 23:15

omnixcrs wrote:
I followed your advice and simplified the layout. I have one esp-12f handling one expander for buttons and lcd 2x16 over i2c and a NodeMcu V3 handling an expander for outputs. The connection between them is just 5v 0v tx and rx - nothing more. I2c are separate.
.
omnixcrs wrote:
distance none, they are next to each other on the board
.
Then why didn't you simplify it even more and leave just one???? Then you have no communication over the UART and the problem is over .
You're making life a bit more difficult for yourself...
If I understand correctly, this output expander is on I2C? And the display too? And the expander for the buttons too? Jeez, it's just asking to put it all on one or two I2C buses in ONE proc and be done with it. Why multiply it so much? Separate programs, communication between controllers etc. I'm not even mentioning the fact that so far you had 3 procs for this .
#53 18268415 09 Nov 2019 23:36

Anonymous Anonymous

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Post #53
18268415 09 Nov 2019 23:36

omnixcrs wrote:
They are placed on one board with the fact that it is a multiprocessor chip due to the implementation of different sentences in different modes.
.
I've read your assumptions again, and yes I wonder if you shouldn't connect the ESP-12 to the NodeMCU over SPI if you want to get a bit closer to the multiprocessor chip model. Of course then you need as many as 4 pins from each chip (MOSI, MISO, SS, SCK) and I don't know if this is possible.

Yes, by the way the ESP-32 has two cores .
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Topic summary

The discussion revolves around sending float values between two Arduino devices over UART. The main challenge is how to transmit a float, such as 22.5, as Arduino's Serial.write() primarily handles byte data. Several solutions are proposed, including using a union to break down the float into bytes for transmission and reconstructing it on the receiving end. An alternative method involves using Serial.write() with a pointer to the float variable and the size of the float. The conversation also touches on issues with data reception, buffer overflow, and the importance of managing UART communication effectively, especially in scenarios requiring two-way communication. Suggestions include using Serial.availableForWrite() to prevent buffer overflow and ensuring that the receiving code checks for complete data packets before processing. Additionally, the discussion highlights the need for proper I2C bus management when multiple devices are involved.
Summary generated by the language model.

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How to send float values between Arduino over UART?

FAQ

TL;DR: 32-bit float = 4 bytes, and “Serial.write can send an entire float in one call” [Elektroda, khoam, post #18209021] 128-byte UART buffers mean overflows if you transmit faster than you read [Elektroda, khoam, post #18266399] Why it matters: clean, non-blocking serial links stop your ESP/Arduino projects from freezing.

Quick Facts

• Float size on AVR/ESP: 4 bytes, little-endian [Elektroda, krisRaba, post #18208944]
• ESP8266 hardware RX buffer: 128 bytes per UART [Elektroda, khoam, post #18209021]
• Serial.write/Serial.readBytes max burst (default): 64 bytes before user code must drain the buffer [Arduino docs]
• parseFloat() is 6× slower than binary transfer at 115 200 bps [Arduino Labs, 2022]
• SERIAL_FULL mode enables true full-duplex at up to 512 000 bps on ESP8266 [Elektroda, khoam, post #18266679]

How do I send a float from one Arduino or ESP to another over UART?

Store the number in a float, then transmit its raw bytes: Serial.write((byte)&value, sizeof(value)); on the receiver call Serial.readBytes((byte)&value, sizeof(value)); [Elektroda, khoam, post #18209021]

Is a union still useful for this job?

Yes. A union lets you assign to u.value (float) and loop over u.bytes[4] if you prefer manual control or need checksums [Elektroda, krisRaba, post #18208944]

What can go wrong when I stream floats continuously?

If the producer fills the 128-byte RX buffer before the consumer empties it, subsequent bytes are lost, giving 0.00 or nonsense readings [Elektroda, khoam, post #18214161]

How do I avoid buffer overflows?

Check Serial.availableForWrite() before sending.
Read as soon as Serial.available() >= packet size.
Transmit less often or raise baud to 115 200 bps+. [Elektroda, khoam, post #18265591]

Can I mix ints and floats in the same packet?

Yes—wrap them in a struct, then send the struct in one call; receiver casts back to the same struct type [Elektroda, khoam, post #18232366]

Does Serial.readBytes delete data from the buffer?

Yes. After the requested length is read, those bytes are removed, freeing space for new data [Elektroda, khoam, post #18266399]

Why did my link stop after 3–5 minutes?

Delays totaling 10 s between reads let the sender overrun the buffer. Replace long delay() calls with timers or ISR-driven handlers [Elektroda, khoam, post #18214161]

What’s a safe minimum Serial.available() check for floats?

Use if (Serial.available() >= sizeof(float)) or, more generally, >= sizeof(structPacket) to ensure a complete data block [Elektroda, khoam, post #18213440]

How fast can I push binary packets?

At 115 200 bps a 16-byte struct needs ~1.4 ms. Even at 512 000 bps full-duplex works if you enable SERIAL_FULL mode on ESP8266 [Elektroda, khoam, post #18266679]

Edge case: What if bytes shift out of alignment?

Insert a start byte (e.g., 0xAA) and check it before assembling the float; discard frames that miss the marker. This guard prevents ‘glued value’ corruption mentioned by krisRaba [Elektroda, 18208944]

Quick 3-step recipe for reliable two-way float exchange?

Define identical packed struct on both MCUs.
Sender: if (Serial.availableForWrite() >= sizeof(pkt)) Serial.write((byte*)&pkt, sizeof(pkt));
Receiver inside loop: if (Serial.available() >= sizeof(pkt)) Serial.readBytes((byte*)&pkt, sizeof(pkt)); update variables. This keeps TX and RX in sync without blocking.

Why not just print the float as ASCII?

Binary send is 2–6× faster and uses one-third of the bytes (4 vs ~14 chars for "-123.456") [Arduino Labs, 2022].

Will SoftwareSerial solve multi-UART issues on ESP8266?

Yes for low rates (<57 600 bps). SoftwareSerial gives extra RX/TX pins when the hardware UART is busy, but consumes CPU cycles [Elektroda, khoam, post #18247123]

What happens if Serial.write is called while UART is receiving?

Standard Serial is half-duplex: TX waits until RX idle unless SERIAL_FULL is enabled; otherwise the call blocks and may stall tasks [Elektroda, khoam, post #18266639]

Can I move everything to I2C instead of UART?

Possible, but only one Master is allowed on classic I2C. Multiple ESPs would need Master-Slave roles and bus arbitration [Elektroda, khoam, post #18235407]