How send a write request?

[smartinick]

Hi there!

i'm trying to connect esp32 to a ble server - i'm able to find the devices, uuid, services, descriptors,...
(btw - using esp32_ble_arduino)

now i need to send a write-request to a handle, but i have no idea how...?

the handle is a BLERemoteDescriptor which is found from a BLERemoteCharacteristic by using the getdescriptor.
using the writevalue method for writing 2 bytes via a char-array (and according to the sniffer it sends those 2 bytes out as desired.

when using the ble sniffer i can watch an android app connect to the ble server and it sends a write-request (OpCode 0x12 in BLE AT Protocoll).
when sniffing on the esp32 it sends the exact same packet except for the opcode which is 0x52 aka write-command.

it does not matter if i set the 3rd parameter from writevalue (response) to true or false, opcode is always 0x52.

searching the sources did not help either, bleremotedescriptor only has read/write value methods but as far as i see there's no writecommand or writerequst methods.

So that's where i'm stuck beeing happy about any usefull hint...

Thanks for your time & Brgds, Martin.

[nkolban]

Howdy,
It should be that given a BLEClient object, you can locate the BLERemoteService and from there locate the BLERemoteCharacteristic which will then take you to the BLERemoteDescriptor. With a reference to a BLERemoteDescriptor object, we should find that we have a method on that called writeValue(). When we invoke writeValue(), that should set the value of the descriptor on the remote BLE server..

Unfortunately, I have never looked at the wire level bits and bytes of BLE (ive so far never had to). The ESP-IDF provides APIs for working with BLE and the cpp_utils BLE classes provide C++ encapsulations of the ESP-IDF BLE APIs.

The actual API that performs the API call looks like:

https://github.com/nkolban/esp32-snippets/blob/master/cpp_utils/BLERemoteDescriptor.cpp#L145

[smartinick]

Hi Neil!

i've used your ble_client example and modified it a bit, in the end i'm doing following:

• Device search, look for a device where adverstisedDevice.getServiceUUID matches
• use advertisedDevice.getAddress in conjunction with a bleclient to connect.
• get the bleremoteservice via client->getservice
• get (totally it's 3 of them) bleremotecharacteristics via bleremoteservice->getcharacteristic
• dump the properties for the remotecharacteristic (canread/write/... and the handle.
• then i get the bleremotedescriptor via bleremotecharacteristic->getdescriptor
• and finally i use the writevalue on the bleremotedescriptor. (and registerfornotification as well).

Now when looking at the blesniffer trace the esp32 sends a packet with the desired payload to the desired handle (aka the same handle/payload that the android-app sends to the bleserver/device).

Except for the small difference mentioned above (BLE-AT OpCode 0x12 vs 0x51).

i can post screenshots or wireshark dump-files if that helps.

but i guess not ;)

i've already read through the bleremotedescriptor/charateristics,... files for your ble library. and also through the esp-idf-arduino sources.

and i'm still clueless as even in espidf sources i can't find a hint how to get the desired opcode sent out.

(by the way, i can write values to the device, reset the esp and read the same value back from the device. so i'd guess the basic communication part is working).

From your feedback i believe i've gone the correct path though not reached the target.... does my feedback give you any hints for anything to test...?

my only idea now is to put this information/questions either in esp32 forum or post a issue with the esp-idf repository - any other ideas?

Thanks &Brgds, Martin.
(And sorry for the delay, just have very sporadic time-resources for programming lately).

[JaspaJami]

I have kind a same problem. Im working with Komoot navigation. Everything else works ok, but it sends only 20 bytes (document speaks of 22 bytes) and when street name is longer i have to ask for more. I just dont know how to write and ask more.

https://github.com/komoot/BLEConnect

[chegewara]

Could you paste that part of code is sending request?

[JaspaJami]

Well i have try to modify this one: https://github.com/SensorsIot/Bluetooth-BLE-on-Arduino-IDE/blob/master/Polar_Receiver/Polar_Receiver.ino

Now if i try to add example that notification enable / disable part to my code ESP32 crash and reboots.

pRemoteCharacteristic->getDescriptor(BLEUUID((uint16_t)0x2902))->writeValue((uint8_t*)notificationOn, 2, true);

[smartinick]

Hm. My Problem was that the BLE-Command beeing sent has the wrong opcode and that there's only a writevalue in the api, no writecommand. it was not about different length and such.

but a hint might be that there's a length limit for ble packets so if your payload and header exceeds that, you'll have to send another packet...

[chegewara]

Now if i try to add example that notification enable / disable part to my code ESP32 crash and reboots.

How logs looks like when it crash?
Try to use this tool to see why it crash:
https://github.com/me-no-dev/EspExceptionDecoder

[smartinick]

i guess either the remoteCharacteristic or the Descriptor returned by getDescriptor is null....
how about this code with nullptr handling?
void SubscribeNotification(BLERemoteCharacteristic* blechar) {
Serial.print("Subscribing to Notification for BLERemoteCharacteristic: ");
static BLERemoteDescriptor* brd;
brd = blechar->getDescriptor(CCC_UUID);
if (brd == nullptr) {
Serial.println("Failed to find CCC Descriptor, ");
} else {
Serial.print("Found CCC Descriptor, sent not_On sequence, ");
brd->writeValue((uint8_t*)not_On,2,true);
}
blechar->registerForNotify(notifyCallback);
Serial.println("registered for notifications!");
}

[smartinick]

@JaspaJami Did you solve the komoot request problem for streets longer fitting in the 1st packet?
Can you share that part somewhere...?
Having the same - clueless - issue now :D

[chegewara]

Did you try to set MTU?

[smartinick]

No. If you look there, they write that they can only send 20 bytes on android and if the street-name is too long it's truncated.

so you "registerfornotify" and get a buffer sent by the phone that is max 20 bytes and if the street name is longer, you need to read the data.... Just reading the value of the bleremotecharacteristic does not do the trick...

and as i rememered JaspaJami asking about this here.... i'm curious if he solved it.

[chegewara]

Just try to use BLEDevice::setMTU(100) for example (up to 512). By default mtu is equal 23 and because of that notify max length is 20 bytes. Im not saying this will work, because peer device may not work with higher mtu values.

[smartinick]

Thanks for the hint, i did a quick test - setMTU does not exist, so i updated the arduino-lib, now BLEDevice does not compile as esp_bt.h does not exist.
so this will take some more time to fix.... i'll come back here with my result!

[smartinick]

So for now give up.
setMTU does not exist on my "old" version of the ble library, so i've updates the lib, which won't compile because esp_bt.h does not exist. created the include file as noted in the other open issue, and now i un into the "c++ eceptions not enabled" issue which is waiting since 2018-02-05....