diff --git a/ili9341.c b/ili9341.c
index e42d6de..abf5a76 100644
--- a/ili9341.c
+++ b/ili9341.c
@@ -29,6 +29,7 @@
 #define DC_DATA			palSetPad(GPIOB, 7)
 
 uint16_t spi_buffer[1024];
+MUTEX_DECL(spi_buffer_mutex);
 
 void
 ssp_wait(void)
@@ -278,6 +279,8 @@ void ili9341_bulk(int x, int y, int w, int h)
 	uint8_t yy[4] = { y >> 8, y, (y+h-1) >> 8, (y+h-1) };
     int len = w * h;
 
+    chMtxLock(&spi_buffer_mutex);
+
 	send_command(0x2A, 4, xx);
 	send_command(0x2B, 4, yy);
 	send_command(0x2C, 0, NULL);
@@ -287,6 +290,8 @@ void ili9341_bulk(int x, int y, int w, int h)
     dmaStreamSetMode(dmatx, txdmamode | STM32_DMA_CR_MINC);
     dmaStreamEnable(dmatx);
     dmaWaitCompletion(dmatx);
+
+    chMtxUnlock(&spi_buffer_mutex);
 }
 #endif
 
@@ -340,6 +345,7 @@ ili9341_drawchar_5x7(uint8_t ch, int x, int y, uint16_t fg, uint16_t bg)
   uint16_t *buf = spi_buffer;
   uint16_t bits;
   int c, r;
+  chMtxLock(&spi_buffer_mutex);
   for(c = 0; c < 7; c++) {
     bits = x5x7_bits[(ch * 7) + c];
     for (r = 0; r < 5; r++) {
@@ -348,6 +354,7 @@ ili9341_drawchar_5x7(uint8_t ch, int x, int y, uint16_t fg, uint16_t bg)
     }
   }
   ili9341_bulk(x, y, 5, 7);
+  chMtxUnlock(&spi_buffer_mutex);
 }
 
 void
@@ -366,6 +373,7 @@ ili9341_drawchar_size(uint8_t ch, int x, int y, uint16_t fg, uint16_t bg, uint8_
   uint16_t *buf = spi_buffer;
   uint16_t bits;
   int c, r;
+  chMtxLock(&spi_buffer_mutex);
   for(c = 0; c < 7*size; c++) {
     bits = x5x7_bits[(ch * 7) + (c / size)];
     for (r = 0; r < 5*size; r++) {
@@ -376,6 +384,7 @@ ili9341_drawchar_size(uint8_t ch, int x, int y, uint16_t fg, uint16_t bg, uint8_
     }
   }
   ili9341_bulk(x, y, 5*size, 7*size);
+  chMtxUnlock(&spi_buffer_mutex);
 }
 
 void
@@ -438,6 +447,7 @@ ili9341_drawfont(uint8_t ch, const font_t *font, int x, int y, uint16_t fg, uint
 	const uint32_t *bitmap = &font->bitmap[font->slide * ch];
 	int c, r, j;
 
+    chMtxLock(&spi_buffer_mutex);
 	for (c = 0; c < font->slide; c++) {
 		for (j = 0; j < font->scaley; j++) {
 			bits = bitmap[c];
@@ -448,6 +458,7 @@ ili9341_drawfont(uint8_t ch, const font_t *font, int x, int y, uint16_t fg, uint
 		}
 	}
     ili9341_bulk(x, y, font->width, font->height);
+    chMtxUnlock(&spi_buffer_mutex);
 }
 
 #if 0
diff --git a/main.c b/main.c
index b18f1bc..f691ec2 100644
--- a/main.c
+++ b/main.c
@@ -52,7 +52,6 @@ int8_t frequency_updated = FALSE;
 int8_t sweep_enabled = TRUE;
 int8_t cal_auto_interpolate = TRUE;
 uint16_t redraw_request = 0; // contains REDRAW_XXX flags
-int8_t stop_the_world = FALSE;
 int16_t vbat = 0;
 
 
@@ -63,11 +62,6 @@ static THD_FUNCTION(Thread1, arg)
     chRegSetThreadName("sweep");
 
     while (1) {
-      if (stop_the_world) {
-          __WFI();
-          continue;
-      }
-
       if (sweep_enabled) {
         chMtxLock(&mutex);
         sweep();
@@ -140,6 +134,8 @@ transform_domain(void)
   // use spi_buffer as temporary buffer
   // and calculate ifft for time domain
   float* tmp = (float*)spi_buffer;
+  chMtxLock(&spi_buffer_mutex);
+  chMtxLock(&mutex);
 
   uint8_t window_size, offset;
   uint8_t is_lowpass = FALSE;
@@ -204,6 +200,8 @@ transform_domain(void)
           }
       }
   }
+  chMtxUnlock(&mutex);
+  chMtxUnlock(&spi_buffer_mutex);
 }
 
 static void cmd_pause(BaseSequentialStream *chp, int argc, char *argv[])
@@ -505,13 +503,9 @@ static void cmd_capture(BaseSequentialStream *chp, int argc, char *argv[])
     (void)argc;
     (void)argv;
 
-    // pause sweep
-    stop_the_world = TRUE;
-
-    chThdSleepMilliseconds(1000);
-
     // use uint16_t spi_buffer[1024] (defined in ili9341) for read buffer
     uint16_t *buf = &spi_buffer[0];
+    chMtxLock(&spi_buffer_mutex);
     int len = 320 * 240;
     int i;
     ili9341_read_memory(0, 0, 320, 240, PART, buf);
@@ -530,8 +524,7 @@ static void cmd_capture(BaseSequentialStream *chp, int argc, char *argv[])
         len -= PART;
     }
     //*/
-
-    stop_the_world = FALSE;
+    chMtxUnlock(&spi_buffer_mutex);
 }
 
 #if 0
@@ -1929,6 +1922,7 @@ int main(void)
     chSysInit();
 
     chMtxObjectInit(&mutex);
+    chMtxObjectInit(&spi_buffer_mutex);
 
     //palSetPadMode(GPIOB, 8, PAL_MODE_ALTERNATE(1) | PAL_STM32_OTYPE_OPENDRAIN);
     //palSetPadMode(GPIOB, 9, PAL_MODE_ALTERNATE(1) | PAL_STM32_OTYPE_OPENDRAIN);
diff --git a/nanovna.h b/nanovna.h
index 6a14fe1..217ceb3 100644
--- a/nanovna.h
+++ b/nanovna.h
@@ -24,6 +24,7 @@
  */
 
 extern float measured[2][101][2];
+extern mutex_t spi_buffer_mutex;
 
 #define CAL_LOAD 0
 #define CAL_OPEN 1
diff --git a/plot.c b/plot.c
index a7167ed..bd348db 100644
--- a/plot.c
+++ b/plot.c
@@ -1153,6 +1153,8 @@ draw_cell(int m, int n)
       grid_mode |= GRID_RECTANGULAR;
   }
 
+  chMtxLock(&spi_buffer_mutex);
+
   PULSE;
   /* draw grid */
   if (grid_mode & GRID_RECTANGULAR) {
@@ -1245,6 +1247,7 @@ draw_cell(int m, int n)
     cell_draw_refpos(m, n, w, h);
 
   ili9341_bulk(OFFSETX + x0off, OFFSETY + y0, w, h);
+  chMtxUnlock(&spi_buffer_mutex);
 }
 
 void
@@ -1528,6 +1531,8 @@ draw_battery_status(void)
     uint16_t *buf = spi_buffer;
     uint8_t vbati = vbat2bati(vbat);
     uint16_t col = vbati == 0 ? RGB565(0, 255, 0) : RGB565(0, 0, 240);
+
+    chMtxLock(&spi_buffer_mutex);
     memset(spi_buffer, 0, w * h * 2);
 
     // battery head
@@ -1587,6 +1592,7 @@ draw_battery_status(void)
         buf[y * w + x++] = col;
 
     ili9341_bulk(0, 1, w, h);
+    chMtxUnlock(&spi_buffer_mutex);
 }
 
 void