Long write only writes 4 packets

[nejcokle]

Hello,

I'm implementing long write function to my project.

I managed to get it working, but i can't send more than 68 bytes of data. When i'm sending the data, I slice it into <18bytes chunks and send each chunk with correct offset, using "BLE_GATT_OP_PREP_WRITE_REQ" as sending type. When all the data is through i use this command to execute the write: "BLE_GATT_OP_EXEC_WRITE_REQ".

Now no matter how much i send (if it's more than 68 bytes), only 68 bytes come through. When i looked into what comes through i saw that only FIRST 4 packets are recieved, even thought i execute when all packets are sent.

Why are other packets lost?

Best regards, Nejc

[Ulrich Myhre]

What is the receiving side? It could have a limited queue, and the GATTS_STATUS in the prepare write response event would contain an error code if that is the case. Likely "Prepare Queue Full".

[nejcokle]

On a receiving side i handle my long write request like that:

case BLE_GATTS_EVT_WRITE:
  LOG_INFO("BLE_GATTS_EVT_WRITE");
break;
case BLE_EVT_USER_MEM_REQUEST:
LOG_INFO("MEM REQUEST");
							
mem_block.len = QUEUED_WRITE_BUFFER_SIZE;
mem_block.p_mem = &queued_write_buffer[0];
							
error_code = sd_ble_user_mem_reply(m_conn_handle, &mem_block);
if (error_code != NRF_SUCCESS) {
LOG_INFO("ERROR sd_ble_user_mem_reply: %u\r\n", (unsigned int)error_code);
} else {
  LOG_INFO("USER_MEM_REQUEST OK\r\n");
}
break;
case BLE_EVT_USER_MEM_RELEASE:
if ((p_ble_evt->evt.common_evt.params.user_mem_release.mem_block.p_mem == mem_block.p_mem)&&(p_ble_evt->evt.common_evt.params.user_mem_release.mem_block.len == mem_block.len))
{
LOG_INFO("USER_MEM_RELEASE\r\n");
}

[nejcokle]

When i monitor my terminal, i get:

1. MEM REQUEST
2. BLE_GATTS_EVT_WRITE
3. USER_MEM_RELEASE

i do not get any errors

[Ulrich Myhre]

Okay, so the receiving side is a Nordic SoftDevice. If you look at the MSCs, you see that you initially get a memory request from the SoftDevice. You reply here with a pointer to a memory block. The size of this block decides how many prepare writes you can store until an execute is needed.

So you need to increase QUEUED_WRITE_BUFFER_SIZE to fit more into the buffer.

[Ulrich Myhre]

Also, it is the sender that will see any errors. When you send a prepare write request, you get a BLE_GATTC_EVT_WRITE_RSP. All the GATTC events have a "gatt_status" flag that contains any error reported by the peer side. In this case, the peer knows that the queue is full, and responds autonomously.