Understanding Zephyr example samples/bluetooth/peripheral

[yayembedded]

I need to build a simple BLE dc motor control with a potentiometer to provide the rotational position of the motor. 

Anyways, my first step is understanding the Nordic libraries.  I opened up the Zephyr peripheral sample and have a few questions.

Here is a code excerpt.

/* Vendor Primary Service Declaration */
BT_GATT_SERVICE_DEFINE(vnd_svc,
 BT_GATT_PRIMARY_SERVICE(&vnd_uuid),
 BT_GATT_CHARACTERISTIC(&vnd_enc_uuid.uuid,
          BT_GATT_CHRC_READ | BT_GATT_CHRC_WRITE |
          BT_GATT_CHRC_INDICATE,
          BT_GATT_PERM_READ_ENCRYPT |
          BT_GATT_PERM_WRITE_ENCRYPT,
          read_vnd, write_vnd, vnd_value),
 BT_GATT_CCC(vnd_ccc_cfg_changed,
      BT_GATT_PERM_READ | BT_GATT_PERM_WRITE_ENCRYPT),
 BT_GATT_CHARACTERISTIC(&vnd_auth_uuid.uuid,
          BT_GATT_CHRC_READ | BT_GATT_CHRC_WRITE,
          BT_GATT_PERM_READ_AUTHEN |
          BT_GATT_PERM_WRITE_AUTHEN,
          read_vnd, write_vnd, vnd_value),
 BT_GATT_CHARACTERISTIC(&vnd_long_uuid.uuid, BT_GATT_CHRC_READ |
          BT_GATT_CHRC_WRITE | BT_GATT_CHRC_EXT_PROP,
          BT_GATT_PERM_READ | BT_GATT_PERM_WRITE |
          BT_GATT_PERM_PREPARE_WRITE,
          read_long_vnd, write_long_vnd, &vnd_long_value),
 BT_GATT_CEP(&vnd_long_cep),
 BT_GATT_CHARACTERISTIC(&vnd_signed_uuid.uuid, BT_GATT_CHRC_READ |
          BT_GATT_CHRC_WRITE | BT_GATT_CHRC_AUTH,
          BT_GATT_PERM_READ | BT_GATT_PERM_WRITE,
          read_signed, write_signed, &signed_value),
 BT_GATT_CHARACTERISTIC(&vnd_write_cmd_uuid.uuid,
          BT_GATT_CHRC_WRITE_WITHOUT_RESP,
          BT_GATT_PERM_WRITE, NULL,
          write_without_rsp_vnd, &vnd_value),
);

static ssize_t read_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
   void *buf, u16_t len, u16_t offset)
{
 const char *value = attr->user_data;

 return bt_gatt_attr_read(conn, attr, buf, len, offset, value,
     strlen(value));
}

static ssize_t write_vnd(struct bt_conn *conn, const struct bt_gatt_attr *attr,
    const void *buf, u16_t len, u16_t offset,
    u8_t flags)
{
 u8_t *value = attr->user_data;

 if (offset + len > sizeof(vnd_value)) {
  return BT_GATT_ERR(BT_ATT_ERR_INVALID_OFFSET);
 }

 memcpy(value + offset, buf, len);

 return len;
}

Question #1

The code defines various custom characteristics for a custom service.  I assume this is just giving an example of what is possible and that those UUID's can be set to any value. Or do vnd_enc and vnd_auth have some specific meaning?

Question #2

Right now reading and writing to those custom attributes are set to callback functions such as read_vnd above.  I see the value to read and return to the client is set from the attr->user_data. However, no value is being read when I try to read that characteristic via the LightBlue app.

What is it's value? When you read a characteristic you do not pass in any arguments so I am confused about this.

Question #3

In the write_vnd function I assume the attr->user_data is the value the client wants to write to this characteristic, but I could be wrong. However, I don't understand why the code

is writing to buf. What is the purpose of it?

[yayembedded]

Question #4

The function below is the callback for the characteristic ending in F3.  When I connect to the device and read this characteristic, I get a really long value.  What is this value? Where is it coming from?  Also, I changed the value of *value = "foo";, but this foo was not returned when I did another read.

static ssize_t read_signed(struct bt_conn *conn, const struct bt_gatt_attr *attr,
      void *buf, u16_t len, u16_t offset)
{
 const char *value = attr->user_data; //"foo";

 return bt_gatt_attr_read(conn, attr, buf, len, offset, value,
     sizeof(signed_value));
}