Can the ESP32-C3 Supermini using the NimBLE Arduino library as a GATT server connect to two phones at the same time?

[jonatanalimin]

As stated on learn.adafruit.com:
“The most important thing to keep in mind with GATT and connections is that connections are exclusive. A BLE peripheral can only be connected to one central device (a mobile phone, etc.) at a time. Once a peripheral connects to a central device, it stops advertising, and other devices will no longer be able to see or connect to it until the existing connection is terminated.”

Is it possible for a peripheral device acting as a GATT server to connect to multiple central devices (such as phones) at the same time?

Update:
When I implemented a non-secure server, I found that the ESP32-C3 supermini could connect to multiple phones.
However, when I enabled MITM protection with a passkey, I noticed that once a new device successfully connects, the previously connected device disconnects with error code 534. Is this behavior expected?

[h2zero]

That's an adafruit limitation. the c3 can connect to up to 9 devices using this library, with the right configuration, default max is 3.

[jonatanalimin]

Hi @h2zero ,
I found that after two phone devices connect to the ESP32-C3 and then disconnect, when they try to reconnect later, the first phone can reconnect immediately without entering the passkey.
However, the second phone gets an error saying “Peer removed pairing information”, and I need to unpair the ESP32-C3 from the phone’s Bluetooth settings before it can connect again.

Is this the expected behavior?

[h2zero]

What are the max bonds and cccd config values set to?

The bond is being erased because there isnt enough room to store the second bond data.

[jonatanalimin]

In nimconfig.h, I didn’t change CONFIG_BT_NIMBLE_MAX_BONDS or CONFIG_BT_NIMBLE_MAX_CCCDS, and I believe the default values are 3 and 8.

In my test, I only used two phones, but after connecting and disconnecting about three times, I found that I needed to unpair first before reconnecting.

Could it be that the address is changing?

[h2zero]

If you only use one phone does this happen?

[jonatanalimin]

Yes, that issue occurred when I was using only one phone.

My NimBLE settings were previously:

NimBLEDevice::setSecurityAuth(true, true, false);
NimBLEDevice::setSecurityPasskey(PASSKEY);
NimBLEDevice::setSecurityIOCap(BLE_HS_IO_DISPLAY_ONLY);
NimBLEDevice::setSecurityInitKey (BLE_SM_PAIR_KEY_DIST_ENC);
NimBLEDevice::setSecurityRespKey(BLE_SM_PAIR_KEY_DIST_ENC);

I then changed them to these:

NimBLEDevice::setSecurityAuth(true, true, false);
NimBLEDevice::setSecurityPasskey(PASSKEY);
NimBLEDevice::setSecurityIOCap(BLE_HS_IO_DISPLAY_ONLY);
NimBLEDevice::setSecurityInitKey (BLE_SM_PAIR_KEY_DIST_ENC | BLE_SM_PAIR_KEY_DIST_ID);
NimBLEDevice::setSecurityRespKey(BLE_SM_PAIR_KEY_DIST_ENC | BLE_SM_PAIR_KEY_DIST_ID);

Now, everything is working normally. Even after restarting the phone or the ESP32-C3, I no longer encounter the pairing error. Is this due to the use of an IRK?