make MTU configurable , needs in board file something like 'DEFINES+=-DNRF_BLE_GATT_MAX_MTU_SIZE=59 -DNRF_BLE_MAX_MTU_SIZE=59', 'LDFLAGS += -Xlinker --defsym=LD_APP_RAM_BASE=0x2d18',

espruino-configurable-MTU.diff

diff --git a/src/jswrap_process.c b/src/jswrap_process.c
index f6c738c3..21470116 100644
--- a/src/jswrap_process.c
+++ b/src/jswrap_process.c
@@ -126,6 +126,9 @@ JsVar *jswrap_process_env() {
 #ifndef SAVE_ON_FLASH
   // Pointer to a list of predefined exports - eventually we'll get rid of the array above
   jsvObjectSetChildAndUnLock(obj, "EXPTR", jsvNewFromInteger((JsVarInt)(size_t)exportPtrs));
+extern uint32_t app_ram_base;
+  jsvObjectSetChildAndUnLock(obj, "APP_RAM_BASE", jsvNewFromInteger((JsVarInt)app_ram_base));
+
 #endif
   return obj;
 }
diff --git a/targetlibs/nrf5x_12/components/ble/ble_services/ble_nus/ble_nus.h b/targetlibs/nrf5x_12/components/ble/ble_services/ble_nus/ble_nus.h
index aeda92f3..536aad46 100644
--- a/targetlibs/nrf5x_12/components/ble/ble_services/ble_nus/ble_nus.h
+++ b/targetlibs/nrf5x_12/components/ble/ble_services/ble_nus/ble_nus.h
@@ -70,7 +70,11 @@ extern "C" {
 #endif
 
 #define BLE_UUID_NUS_SERVICE 0x0001                      /**< The UUID of the Nordic UART Service. */
+#ifdef NRF_BLE_MAX_MTU_SIZE
+#define BLE_NUS_MAX_DATA_LEN (NRF_BLE_MAX_MTU_SIZE - 3) /**< Maximum length of data (in bytes) that can be transmitted to the peer by the Nordic UART service module. */
+#else
 #define BLE_NUS_MAX_DATA_LEN (GATT_MTU_SIZE_DEFAULT - 3) /**< Maximum length of data (in bytes) that can be transmitted to the peer by the Nordic UART service module. */
+#endif
 
 /* Forward declaration of the ble_nus_t type. */
 typedef struct ble_nus_s ble_nus_t;
diff --git a/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino.ld b/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino.ld
index 7e0ef8a6..e1fa87c0 100644
--- a/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino.ld
+++ b/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino.ld
@@ -4,7 +4,7 @@ SEARCH_DIR(.)
 GROUP(-lgcc -lc -lnosys)
 
 MEM_SRAM_SIZE = DEFINED( LD_SRAM_SIZE ) ? LD_SRAM_SIZE : 0x10000 ;
-MEM_SRAM_SD_OFFSET = 0x2c40; /* memory allocated for SoftDevice */
+MEM_SRAM_SD_OFFSET = DEFINED( LD_APP_RAM_BASE ) ? LD_APP_RAM_BASE :0x2c40; /* memory allocated for SoftDevice */
 MEM_NOINIT_SIZE = 16;
 
 MEMORY
diff --git a/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino_bootloader.ld b/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino_bootloader.ld
index b65ca893..3ca2c35f 100644
--- a/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino_bootloader.ld
+++ b/targetlibs/nrf5x_12/nrf5x_linkers/linker_nrf52_ble_espruino_bootloader.ld
@@ -3,11 +3,15 @@
 SEARCH_DIR(.)
 GROUP(-lgcc -lc -lnosys)
 
+MEM_SRAM_SIZE = DEFINED( LD_SRAM_SIZE ) ? LD_SRAM_SIZE : 0x10000 ;
+MEM_SRAM_SD_OFFSET = DEFINED( LD_APP_RAM_BASE ) ? LD_APP_RAM_BASE :0x2c40; /* memory allocated for SoftDevice */
+MEM_NOINIT_SIZE = 16;
+
 MEMORY
 {
   FLASH (rx) : ORIGIN = 0x1f000, LENGTH = 0x55000 /* 0x77000 and above is reserved for the bootloader+fs, then there's saved JS code. see Makefile, secure_dfu_gcc_nrf52.ld,  linker_nrf52_ble_espruino_bootloader.ld and dfu_types.h, BOARD.py  */
-  RAM (rwx) :  ORIGIN = 0x20002c40, LENGTH = 0xd3b0
-  NOINIT (rwx) :  ORIGIN = 0x2000fff0, LENGTH = 16
+  RAM (rwx) :  ORIGIN = 0x20000000 + MEM_SRAM_SD_OFFSET, LENGTH = MEM_SRAM_SIZE - MEM_SRAM_SD_OFFSET - MEM_NOINIT_SIZE
+  NOINIT (rwx) :  ORIGIN = 0x20000000 + MEM_SRAM_SIZE - MEM_NOINIT_SIZE , LENGTH = MEM_NOINIT_SIZE
 }
 
 SECTIONS
diff --git a/targetlibs/nrf5x_15/nrf5x_linkers/linker_nrf52840_ble_espruino.ld b/targetlibs/nrf5x_15/nrf5x_linkers/linker_nrf52840_ble_espruino.ld
index 36b05619..258c6ffd 100644
--- a/targetlibs/nrf5x_15/nrf5x_linkers/linker_nrf52840_ble_espruino.ld
+++ b/targetlibs/nrf5x_15/nrf5x_linkers/linker_nrf52840_ble_espruino.ld
@@ -3,15 +3,15 @@
 SEARCH_DIR(.)
 GROUP(-lgcc -lc -lnosys)
 
+MEM_SRAM_SIZE = DEFINED( LD_SRAM_SIZE ) ? LD_SRAM_SIZE : 0x40000 ;
+MEM_SRAM_SD_OFFSET = DEFINED( LD_APP_RAM_BASE ) ? LD_APP_RAM_BASE :0x3c8d0; /* FIXME - can do way less */ /* memory allocated for SoftDevice */
+MEM_NOINIT_SIZE = 24;
+
 MEMORY
 {
   FLASH (rx) : ORIGIN = 0x26000, LENGTH = 0xda000
-  RAM (rwx) :  ORIGIN = 0x20003720, LENGTH = 0x3c8d0 /* FIXME - can do way less */
-  NOINIT (rwx) :  ORIGIN = 0x2003fff0, LENGTH = 16
-}
-
-SECTIONS
-{
+  RAM (rwx) :  ORIGIN = 0x20000000 + MEM_SRAM_SD_OFFSET, LENGTH = MEM_SRAM_SIZE - MEM_SRAM_SD_OFFSET - MEM_NOINIT_SIZE
+  NOINIT (rwx) :  ORIGIN = 0x20000000 + MEM_SRAM_SIZE - MEM_NOINIT_SIZE , LENGTH = MEM_NOINIT_SIZE
 }
 
 SECTIONS
diff --git a/targets/nrf5x/bluetooth.c b/targets/nrf5x/bluetooth.c
index 27df1212..b9b2067b 100644
--- a/targets/nrf5x/bluetooth.c
+++ b/targets/nrf5x/bluetooth.c
@@ -97,11 +97,16 @@ __ALIGN(4) static ble_gap_lesc_dhkey_t m_lesc_dhkey;   /**< LESC ECC DH Key*/
 // -----------------------------------------------------------------------------------
 
 #if NRF_SD_BLE_API_VERSION < 5
+#ifndef NRF_BLE_MAX_MTU_SIZE
 #define NRF_BLE_MAX_MTU_SIZE            GATT_MTU_SIZE_DEFAULT                        /**< MTU size used in the softdevice enabling and to reply to a BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event. */
+#endif
 #define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(100, APP_TIMER_PRESCALER)   /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
 #define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(10000, APP_TIMER_PRESCALER) /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */
 #define MAX_CONN_PARAMS_UPDATE_COUNT    3                                           /**< Number of attempts before giving up the connection parameter negotiation. */
 #else
+#ifndef GATT_MTU_SIZE_DEFAULT
+#define GATT_MTU_SIZE_DEFAULT  BLE_GATT_ATT_MTU_DEFAULT
+#endif
 #define NRF_BLE_MAX_MTU_SIZE            NRF_SDH_BLE_GATT_MAX_MTU_SIZE               /**< MTU size used in the softdevice enabling and to reply to a BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST event. */
 #define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(100)  /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
 #define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(10000) /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */
@@ -170,8 +175,10 @@ static uint8_t m_adv_handle = BLE_GAP_ADV_SET_HANDLE_NOT_SET;
 #endif
 
 volatile uint16_t                       m_peripheral_conn_handle = BLE_CONN_HANDLE_INVALID;    /**< Handle of the current connection. */
+volatile uint16_t m_peripheral_effective_mtu;
 #if CENTRAL_LINK_COUNT>0
 volatile uint16_t                       m_central_conn_handle = BLE_CONN_HANDLE_INVALID; /**< Handle for central mode connection */
+volatile uint16_t m_central_effective_mtu;
 #endif
 #ifdef USE_NFC
 volatile bool nfcEnabled = false;
@@ -965,6 +972,7 @@ void nus_transmit_string() {
    * do 5, but it seems some things have issues with
    * this (eg nRF cloud gateways) so only send 1 packet
    * for now. */
+  int max_data_len = MIN((m_peripheral_effective_mtu-3),BLE_NUS_MAX_DATA_LEN);
   for (int packet=0;packet<1;packet++) {
     // No data? try and get some from our queue
     if (!nuxTxBufLength) {
@@ -972,7 +980,7 @@ void nus_transmit_string() {
       int ch = jshGetCharToTransmit(EV_BLUETOOTH);
       while (ch>=0) {
         nusTxBuf[nuxTxBufLength++] = ch;
-        if (nuxTxBufLength>=BLE_NUS_MAX_DATA_LEN) break;
+        if (nuxTxBufLength>=max_data_len) break;
         ch = jshGetCharToTransmit(EV_BLUETOOTH);
       }
     }
@@ -1118,6 +1126,7 @@ static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context) {
       case BLE_GAP_EVT_CONNECTED:
         if (p_ble_evt->evt.gap_evt.params.connected.role == BLE_GAP_ROLE_PERIPH) {
           m_peripheral_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
+          m_peripheral_effective_mtu = GATT_MTU_SIZE_DEFAULT;
 #ifdef DYNAMIC_INTERVAL_ADJUSTMENT
           bleIdleCounter = 0;
 #endif
@@ -1135,6 +1144,13 @@ static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context) {
 #if CENTRAL_LINK_COUNT>0
         if (p_ble_evt->evt.gap_evt.params.connected.role == BLE_GAP_ROLE_CENTRAL) {
           m_central_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
+          m_central_effective_mtu = GATT_MTU_SIZE_DEFAULT;
+#if NRF_SD_BLE_API_VERSION>=3
+#if (NRF_BLE_MAX_MTU_SIZE > GATT_MTU_SIZE_DEFAULT)
+          err_code = sd_ble_gattc_exchange_mtu_request(p_ble_evt->evt.gap_evt.conn_handle, NRF_BLE_MAX_MTU_SIZE);
+          APP_ERROR_CHECK_NOT_URGENT(err_code);
+#endif
+#endif
           bleSetActiveBluetoothGattServer(bleTaskInfo);
           jsble_queue_pending(BLEP_TASK_CENTRAL_CONNECTED, 0);
         }
@@ -1304,7 +1320,47 @@ static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context) {
       } break; // BLE_GATTS_EVT_RW_AUTHORIZE_REQUEST
 
 #if (NRF_SD_BLE_API_VERSION >= 3)
+
+        case BLE_GATTC_EVT_EXCHANGE_MTU_RSP: {
+          uint16_t conn_handle   = p_ble_evt->evt.gattc_evt.conn_handle;
+          uint16_t effective_mtu = p_ble_evt->evt.gattc_evt.params.exchange_mtu_rsp.server_rx_mtu;
+          effective_mtu = MIN(MAX(GATT_MTU_SIZE_DEFAULT,effective_mtu),NRF_BLE_MAX_MTU_SIZE);
+#if CENTRAL_LINK_COUNT>0
+          if (m_central_conn_handle == conn_handle) {
+                m_central_effective_mtu = effective_mtu;
+#if (NRF_SD_BLE_API_VERSION > 3)
+                if (effective_mtu > GATT_MTU_SIZE_DEFAULT){
+                    ble_gap_data_length_params_t const dlp =
+                    {
+                        .max_rx_octets =  effective_mtu + 4,
+                        .max_tx_octets =  effective_mtu + 4,
+                    };
+                    err_code = sd_ble_gap_data_length_update(conn_handle, &dlp, NULL);
+                    APP_ERROR_CHECK_NOT_URGENT(err_code);
+
+                }
+#endif
+          } else
+#endif
+          if (m_peripheral_conn_handle == conn_handle){
+                 m_peripheral_effective_mtu = effective_mtu;
+          }
+        } break; // BLE_GATTC_EVT_EXCHANGE_MTU_RSP
+
+
       case BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST: {
+        uint16_t conn_handle   = p_ble_evt->evt.gatts_evt.conn_handle;
+        uint16_t effective_mtu = p_ble_evt->evt.gatts_evt.params.exchange_mtu_request.client_rx_mtu;
+        effective_mtu = MIN(MAX(GATT_MTU_SIZE_DEFAULT,effective_mtu),NRF_BLE_MAX_MTU_SIZE);
+#if CENTRAL_LINK_COUNT>0
+        if (m_central_conn_handle == conn_handle) {
+                 m_central_effective_mtu = effective_mtu;
+        } else
+#endif
+        if (m_peripheral_conn_handle == conn_handle){
+                 m_peripheral_effective_mtu = effective_mtu;
+        }
+
         err_code = sd_ble_gatts_exchange_mtu_reply(p_ble_evt->evt.gatts_evt.conn_handle,
                                                    NRF_BLE_MAX_MTU_SIZE);
         // This can return an error when connecting to EQ3 CC-RT-BLE (which requests an MTU of 0!!)
@@ -2246,8 +2302,17 @@ static void services_init() {
 #endif
 }
 
+uint32_t app_ram_base;
+
 /// Function for the SoftDevice initialization.
 static void ble_stack_init() {
+#if defined ( __GNUC__ )
+    extern uint32_t __data_start__;
+    app_ram_base = (uint32_t) &__data_start__;
+#else
+#error "unsupported compiler"
+#endif
+
 #if NRF_SD_BLE_API_VERSION<5
 
     uint32_t err_code;
@@ -2289,9 +2354,21 @@ static void ble_stack_init() {
 #if (NRF_SD_BLE_API_VERSION >= 3)
     ble_enable_params.gatt_enable_params.att_mtu = NRF_BLE_MAX_MTU_SIZE;
 #endif
-    err_code = softdevice_enable(&ble_enable_params);
+
+    err_code = sd_ble_enable(&ble_enable_params,&app_ram_base);
     APP_ERROR_CHECK(err_code);
 
+#if (NRF_BLE_MAX_MTU_SIZE > GATT_MTU_SIZE_DEFAULT)
+    {
+        ble_opt_t gap_opt;
+        gap_opt.gap_opt.ext_len.rxtx_max_pdu_payload_size = NRF_BLE_MAX_MTU_SIZE+4;
+        err_code = sd_ble_opt_set(BLE_GAP_OPT_EXT_LEN, &gap_opt);
+        APP_ERROR_CHECK(err_code);
+        gap_opt.common_opt.conn_evt_ext.enable = 1;
+        err_code = sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &gap_opt); // enable DLE
+        APP_ERROR_CHECK(err_code);
+    }
+#endif
     // Subscribe for BLE events.
     err_code = softdevice_ble_evt_handler_set(ble_evt_dispatch);
     APP_ERROR_CHECK(err_code);
@@ -2312,9 +2389,19 @@ static void ble_stack_init() {
     uint32_t ram_start = 0;
     err_code = nrf_sdh_ble_default_cfg_set(APP_BLE_CONN_CFG_TAG, &ram_start);
     APP_ERROR_CHECK(err_code);
+#if (NRF_BLE_MAX_MTU_SIZE > GATT_MTU_SIZE_DEFAULT)
+    {
+        ble_cfg_t cfg;
+        memset(&cfg, 0, sizeof(ble_cfg_t));
+        cfg.conn_cfg.conn_cfg_tag = APP_BLE_CONN_CFG_TAG;
+        cfg.conn_cfg.params.gatt_conn_cfg.att_mtu = NRF_BLE_MAX_MTU_SIZE;
+        err_code = sd_ble_cfg_set(BLE_CONN_CFG_GATT, &cfg, app_ram_base);
+        APP_ERROR_CHECK(err_code);
+    }
+#endif
 
     // Enable BLE stack.
-    err_code = nrf_sdh_ble_enable(&ram_start);
+    err_code = nrf_sdh_ble_enable(&app_ram_base);
     APP_ERROR_CHECK(err_code);
 
     // Register a handler for BLE events.