Error while configuring cccd metadata, in ble-characteristics-a-beginners-tutorial

[Geoffroy]

I'm using nRF52840 with SDK15.0.0

Following this tutorial : https://devzone.nordicsemi.com/tutorials/b/bluetooth-low-energy/posts/ble-characteristics-a-beginners-tutorial

AT STEP 2.H my module is Advertising i can connect and then see the service and its characteristics write and read with nRF connect app.

Then at  next  step :STEP 3.A, i add : 

		BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
		BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);
		cccd_md.vloc                = BLE_GATTS_VLOC_STACK;
		char_md.p_cccd_md           = &cccd_md;
		char_md.char_props.notify   = 1;

Here is problem : there is err_code 0x07 in return from sd_ble_gatts_characteristic_add, and no more advertising.

if i comment APP_ERROR_CHECK where err_code = 0x07, i can see my module advertising but i don't see the service and its characteritics

I tryed to use solutions file, but i get same problem...

[Geoffroy]

if i comment this line : 

char_md.p_cccd_md           = &cccd_md;

err_code is 0x00

i don't know if it can help, but i printed char_md.p_cccd_md value : 0x2000FEBC,

in file ble_app_template_gcc_nrf52.ld i have values : 

MEMORY
{
  FLASH (rx) : ORIGIN = 0x26000, LENGTH = 0xda000
  RAM (rwx) :  ORIGIN = 0x20002274, LENGTH = 0xdd8c
}

I tryed to declare cc_md in static : 

static ble_gatts_attr_md_t cccd_md;

here i get char_md.p_cccd_md value : 0x20002F00, i still have err_code 0x07

[awneil]

Continued from previous thread:

https://devzone.nordicsemi.com/f/nordic-q-a/46981/characteristic-tutorial-error-when-adding-a-characteristic-to-a-service/186578#186578

[Geoffroy]

i just checked values : 

		SEGGER_RTT_printf(0, "read_perm : 0x%#02x\n", char_md.p_cccd_md->read_perm);
		SEGGER_RTT_printf(0, "write_perm : 0x%#02x\n", char_md.p_cccd_md->write_perm);

and i get read_perm : 0x11 and write_perm : 0x11 it seems good.

[Geoffroy]

My problem seems to be solve !!!  

in ble_gatts.h, because of https://devzone.nordicsemi.com/f/nordic-q-a/46739/device-crash-when-adding-new-ble-service-ble-services-beginner-tutorial, i had the code : 

#define BLE_GATTS_VLOC_INVALID       0x00
#define BLE_GATTS_VLOC_STACK         0x02
#define BLE_GATTS_VLOC_USER          0x02

i changed to : 

#define BLE_GATTS_VLOC_INVALID       0x00
#define BLE_GATTS_VLOC_STACK         0x01
#define BLE_GATTS_VLOC_USER          0x01

then no more errors ...

I'll do few more verifications to check all is fine compared to this tutorial...

[Marjeris Romero]

Hi Geoffroy,

Please don't change the header files. It would make it impossible for you to keep track of which values you are using then. The correct values are

/** @defgroup BLE_GATTS_VLOCS GATT Value Locations
 * @{ */
#define BLE_GATTS_VLOC_INVALID       0x00  /**< Invalid Location. */
#define BLE_GATTS_VLOC_STACK         0x01  /**< Attribute Value is located in stack memory, no user memory is required. */
#define BLE_GATTS_VLOC_USER          0x02  /**< Attribute Value is located in user memory. This requires the user to maintain a valid buffer through the lifetime of the attribute, since the stack
                                                will read and write directly to the memory using the pointer provided in the APIs. There are no alignment requirements for the buffer. */

If you want the attribute data stored in the stack memory you set

	cccd_md.vloc                = BLE_GATTS_VLOC_STACK;

instead of having to set it to 0x01.

If you want the attribute data stored in user memory you set

	cccd_md.vloc                = BLE_GATTS_VLOC_USER;

instead of having to set it to 0x02.

Does this makes sense? You are not suppose to change the header files, just use them for reference.

When you changed the macro for BLE_GATTS_VLOCK_STACK to 0x02, you were actually defining the attribute value location to be BLE_GATTS_VLOCK_USER.

As you can read from the comments in the header file if you choose BLE_GATTS_VLOCK_USER the attribute value is located in user memory, but this requires the user to mantain a valid buffer through the lifetime of the attribute and you need to store the data in the buffer yourself, so this was why the example was not working.

Best regards,

Marjeris