diff --git a/firmware/target/arm/as3525/pcm-as3525.c b/firmware/target/arm/as3525/pcm-as3525.c
index 26e018c..7d3ba94 100644
--- a/firmware/target/arm/as3525/pcm-as3525.c
+++ b/firmware/target/arm/as3525/pcm-as3525.c
@@ -220,6 +220,7 @@ void pcm_rec_dma_stop(void)
 }
 
 
+#if 0   /* C */
 void INT_I2SIN(void)
 {
     register int status;
@@ -255,6 +256,70 @@ void INT_I2SIN(void)
         }
     }
 }
+#else   /* ASM */
+void INT_I2SIN(void) __attribute__((naked));
+void INT_I2SIN(void)
+{
+    asm volatile(
+    "   str     lr, [sp, #-4]!                      \n"
+
+    "   ldr r0, =0xC8090008     @ I2SIN_RAW_STATUS  \n"
+    "   ldrb    r1, [r0, #4]    @ I2SIN_STATUS      \n"
+    "   strb    r1, [r0, #8]    @ I2SIN_CLEAR       \n"
+
+    "   ldr r2, =rec_size                           \n"
+    "   ldr r3, =rec_start_addr                     \n"
+
+    "   ldrb    r1, [r0]                            \n"
+    "   tst r1, #(1<<5)     @ test fifo pop empty   \n"
+    "   bne 3f @no_samples                          \n"
+
+    "   ldr r1, [r2]        @ test if rec_size == 0 \n"
+    "   cmp r1, #0                                  \n"
+    "   beq 4f @rec_size_null                       \n"
+
+    "   ldr r12, [r3]       @ load rec_start_addr   \n"
+    "   ldr lr, [r2]        @ load rec_size         \n"
+
+    "1: @loop                                       \n"
+    "   ldr r1, [r0, #0xC]  @ read I2SIN_DATA       \n"
+    "   str r1, [r12], #4   @ store samples         \n"
+
+    "   sub lr, lr, #4      @ decrement rec_size    \n"
+    "   beq 2f @end_loop    @ exit loop if rec_size is 0    \n"
+
+    "   ldrb    r1, [r0]    @ I2SIN_RAW_STATUS      \n"
+    "   tst r1, #(1<<5)     @ test fifo pop empty   \n"
+    "   beq 1b @loop                                \n"
+
+    "2: @end_loop                                   \n"
+    "   str r12, [r3]       @ store new rec_start_addr  \n"
+    "   str lr, [r2]        @ store new rec_size    \n"
+
+    "3: @no_samples                                 \n"
+    "   ldr r0, [r2]        @ return if rec_size > 0    \n"
+    "   cmp r0, #0                                  \n"
+    "   bne 6f @ret                                 \n"
+
+    "4: @rec_size_null                              \n"
+    "   ldr r3, =pcm_callback_more_ready            \n"
+    "   ldr r3, [r3]                                \n"
+    "   cmp r3, #0                                  \n"
+    "   beq 5f @stop_record                         \n"
+    "   mov lr, pc                                  \n"
+    "   bx  r3                                      \n"
+    "   cmp r0, #0                                  \n"
+    "   bge 6f @ret                                 \n"
+
+    "5: @stop_record                                \n"
+    "   bl  pcm_rec_dma_stop                        \n"
+    "   bl  pcm_rec_dma_stopped_callback            \n"
+
+    "6: @ret                                        \n"
+    "   ldr pc, [sp], #4                            \n"
+    : : : "r0", "r1", "r2", "r3", "r12", "lr");
+}
+#endif
 
 
 void pcm_rec_dma_start(void *addr, size_t size)