ble_app_hids_keyboard: why send_key_scan_press_release has to loop?

Question

Hi guys. May I ask one firmware code question for ble_app_hids_keyboard_pca10056_s113 project from 17.1.0 nRF5 SDK: 
 static uint8_t m_caps_on_key_scan_str[] = /**< Key pattern to be sent when the output report has been written with the CAPS LOCK bit set. */
{
 0x06, /* Key C */
 0x04, /* Key a */
 0x13, /* Key p */
 0x16, /* Key s */
 0x12, /* Key o */
 0x11, /* Key n */
};

keys_send(sizeof(m_caps_on_key_scan_str), m_caps_on_key_scan_str);
// this calls send_key_scan_press_release()

static uint32_t send_key_scan_press_release(){
do{
ble_hids_inp_rep_send(); // Send C a p s o n
}while (offset <= data_len);
}
}

When sending "C a p s o n", this uses a DO-WHILE LOOP. The data becomes: 
 1. 00 00 06 04 13 16 12 11 
 2. 00 00 00 04 13 16 12 11 
 3. 00 00 00 00 13 16 12 11 
 4. 00 00 00 00 00 16 12 11 
 5. 00 00 00 00 00 00 12 11 
 6. 00 00 00 00 00 00 00 11 
 7. 00 00 00 00 00 00 00 00 
 As explained in the INFO center https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.1.0/ble_sdk_app_hids_keyboard.html

May I ask why this DO-WHILE loop is needed? Can't we send these two data instead: 
 1. 00 00 06 04 13 16 12 11 
 7. 00 00 00 00 00 00 00 00 
 Skipping the DO-WHILE step 2 to 6? Thank you so much for your help!

Håkon Alseth · Accepted Answer

Hi, 
 
 This is the code in the sample: 
 static uint32_t send_key_scan_press_release(ble_hids_t * p_hids,
 uint8_t * p_key_pattern,
 uint16_t pattern_len,
 uint16_t pattern_offset,
 uint16_t * p_actual_len)
{
 ret_code_t err_code;
 uint16_t offset;
 uint16_t data_len;
 uint8_t data[INPUT_REPORT_KEYS_MAX_LEN];

// HID Report Descriptor enumerates an array of size 6, the pattern hence shall not be any
 // longer than this.
 STATIC_ASSERT((INPUT_REPORT_KEYS_MAX_LEN - 2) == 6);

ASSERT(pattern_len <= (INPUT_REPORT_KEYS_MAX_LEN - 2));

offset = pattern_offset;
 data_len = pattern_len;

do
 {
 // Reset the data buffer.
 memset(data, 0, sizeof(data));

// Copy the scan code.
 memcpy(data + SCAN_CODE_POS + offset, p_key_pattern + offset, data_len - offset);

if (bsp_button_is_pressed(SHIFT_BUTTON_ID))
 {
 data[MODIFIER_KEY_POS] |= SHIFT_KEY_CODE;
 }

if (!m_in_boot_mode)
 {
 err_code = ble_hids_inp_rep_send(p_hids,
 INPUT_REPORT_KEYS_INDEX,
 INPUT_REPORT_KEYS_MAX_LEN,
 data,
 m_conn_handle);
 }
 else
 {
 err_code = ble_hids_boot_kb_inp_rep_send(p_hids,
 INPUT_REPORT_KEYS_MAX_LEN,
 data,
 m_conn_handle);
 }

if (err_code != NRF_SUCCESS)
 {
 break;
 }

offset++;
 }
 while (offset <= data_len);

*p_actual_len = offset;

return err_code;
} 
 
 This will send one-and-one char, which again is the default use-case when you are using a keyboard as well. 
 If you send the data in one payload, it will be like pressing all the buttons at the exact same time. It will then depend on the host implementation on which way you start to parse the data, ie: "Capson" or "nospaC". 
 
 Kind regards, 
 Håkon

ble_app_hids_keyboard: why send_key_scan_press_release has to loop?

Top Replies