mtu size unclear
thijses opened this issue · 3 comments
I'm trying to determine what the maximum package size for my STM32WB55 based application should be.
- CFG_BLE_MAX_ATT_MTU defaults to 156
(but SHCI_C2_BLE_INIT_OPTIONS_LL_ONLY is enabled, so this can be overridden???) - using an ST-link debugging session, ATTClass._maxMtu (and the _peers[0].mtu) read as 242 (which is readLeBufferSize() pktLen (251) - 9. Presumably determined by CPU2 in some way, right?)
- anecdotally the largest packet that the MCU acknowledges is 115 bytes (both from ST BLE Toolbox app and NFR Connect app)
so which one is it? (and which macro/function can provide the correct number in the future?)
P.S.
i am trying to make a basic OTA application, but the ST BLE Toolbox app insists on using 244 byte packets. I know that BLE 5.0 can technically handle an MTU of 512(ish?), but i'd be willing to wait a little longer and just use the 20-byte packet structure of BLE 4.0 if that were an option.
Hi @thijses
It is not possible to change it as stated here:
https://github.com/stm32duino/STM32duinoBLE/blob/1c3d6070189e813b4edbc4aaf3aa52717e06c9cf/src/utility/STM32_WPAN/app_conf_default.h#L177C16-L177C16
The MTU size is defined by the CPU2.
Thank you for your reply (and sorry for the mislabelled message).
Is there currently any public class member I can read out to find the maximum data packet size (after MTU negotiation with the BLE counterpart)?
Probably, but don't know when it is, you can try to enable the core debug and log messaged.