diff --git a/main.c b/main.c
index 94c2b74..24b2351 100644
--- a/main.c
+++ b/main.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2015, TAKAHASHI Tomohiro (TTRFTECH) edy555@gmail.com
+ * Copyright (c) 2016-2017, TAKAHASHI Tomohiro (TTRFTECH) edy555@gmail.com
  * All rights reserved.
  *
  * This is free software; you can redistribute it and/or modify
@@ -39,7 +39,7 @@ static MUTEX_DECL(mutex);
 int32_t frequency_offset = 5000;
 int32_t frequency = 10000000;
 uint8_t drive_strength = SI5351_CLK_DRIVE_STRENGTH_2MA;
-
+int8_t frequency_updated = FALSE;
 
 static THD_WORKING_AREA(waThread1, 440);
 static THD_FUNCTION(Thread1, arg)
@@ -376,11 +376,15 @@ void scan_lcd(void)
   int delay;
   //int first = TRUE;
 
+ rewind:
+  frequency_updated = FALSE;
+
   delay = set_frequency(frequencies[0]);
   delay += 2;
   for (i = 0; i < sweep_points; i++) {
     tlv320aic3204_select_in3();
     wait_dsp(delay+2);
+    // blink LED while scanning
     palClearPad(GPIOC, GPIOC_LED);
 
     /* calculate reflection coeficient */
@@ -393,8 +397,12 @@ void scan_lcd(void)
     calculate_gamma(measured[1][i]);
 
     delay = set_frequency(frequencies[(i+1)%sweep_points]);
+    // blink LED while scanning
     palSetPad(GPIOC, GPIOC_LED);
     ui_process();
+
+    if (frequency_updated)
+      goto rewind;
   }
 
   if (cal_status & CALSTAT_APPLY)
@@ -435,6 +443,7 @@ update_frequencies(void)
   for (i = 0; i < sweep_points; i++)
     frequencies[i] = start + span * i / (sweep_points - 1) * 100;
 
+  frequency_updated = TRUE;
   // set grid layout
   update_grid();
 }
diff --git a/ui.c b/ui.c
index 867dc03..607f75b 100644
--- a/ui.c
+++ b/ui.c
@@ -69,7 +69,7 @@ enum {
 
 uint8_t ui_mode = UI_NORMAL;
 uint8_t keypad_mode;
-uint8_t selection = 0;
+int8_t selection = 0;
 
 typedef struct {
   uint8_t type;
@@ -471,14 +471,16 @@ static void
 menu_stimulus_cb(int item)
 {
   switch (item) {
-  case 0:
-  case 1:
-  case 2:
-  case 3:
-  case 4:
+  case 0: /* START */
+  case 1: /* STOP */
+  case 2: /* CENTER */
+  case 3: /* SPAN */
+  case 4: /* CW */
     ui_mode_keypad(item);
     ui_process_keypad();
     break;
+  case 5: /* pause && resume */
+    break;
   }
 }
 
@@ -583,6 +585,7 @@ const menuitem_t menu_stimulus[] = {
   { MT_CALLBACK, "CENTER", menu_stimulus_cb },
   { MT_CALLBACK, "SPAN", menu_stimulus_cb },
   { MT_CALLBACK, "CW", menu_stimulus_cb },
+  { MT_CALLBACK, "PAUSE", menu_stimulus_cb },
   { MT_CANCEL, "BACK", NULL },
   { MT_NONE, NULL, NULL } // sentinel
 };
@@ -855,6 +858,7 @@ ui_mode_menu(void)
     return;
 
   ui_mode = UI_MENU;
+  /* narrowen plotting area */
   area_width = WIDTH - (64-14-4);
   area_height = HEIGHT;
   ensure_selection();
@@ -1090,6 +1094,7 @@ void ui_process_touch(void)
     switch (ui_mode) {
     case UI_NORMAL:
       touch_wait_release();
+      selection = -1;
       ui_mode_menu();
       break;