diff --git a/protocol/serial.h b/protocol/serial.h
index bd071bec..96913c86 100644
--- a/protocol/serial.h
+++ b/protocol/serial.h
@@ -41,6 +41,7 @@ POSSIBILITY OF SUCH DAMAGE.
 /* host role */
 void serial_init(void);
 uint8_t serial_recv(void);
+int16_t serial_recv2(void);
 void serial_send(uint8_t data);
 
 #endif
diff --git a/protocol/serial_soft.c b/protocol/serial_soft.c
index beddc353..3c9c914e 100644
--- a/protocol/serial_soft.c
+++ b/protocol/serial_soft.c
@@ -48,8 +48,20 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #define WAIT_US     (1000000/SERIAL_BAUD)
 
+/* debug for signal timing, see debug pin with oscilloscope */
+#ifdef SERIAL_SOFT_DEBUG
+    #define SERIAL_SOFT_DEBUG_INIT()    (DDRD |= 1<<7)
+    #define SERIAL_SOFT_DEBUG_TGL()     (PORTD ^= 1<<7)
+#else
+    #define SERIAL_SOFT_DEBUG_INIT()
+    #define SERIAL_SOFT_DEBUG_TGL()
+#endif
+
+
 void serial_init(void)
 {
+    SERIAL_SOFT_DEBUG_INIT();
+
     SERIAL_RXD_INIT();
     SERIAL_TXD_INIT();
 }
@@ -60,6 +72,7 @@ static uint8_t rbuf[RBUF_SIZE];
 static uint8_t rbuf_head = 0;
 static uint8_t rbuf_tail = 0;
 
+
 uint8_t serial_recv(void)
 {
     uint8_t data = 0;
@@ -72,6 +85,18 @@ uint8_t serial_recv(void)
     return data;
 }
 
+int16_t serial_recv2(void)
+{
+    uint8_t data = 0;
+    if (rbuf_head == rbuf_tail) {
+        return -1;
+    }
+
+    data = rbuf[rbuf_tail];
+    rbuf_tail = (rbuf_tail + 1) % RBUF_SIZE;
+    return data;
+}
+
 void serial_send(uint8_t data)
 {
     /* signal state: IDLE: ON, START: OFF, STOP: ON, DATA0: OFF, DATA1: ON */
@@ -103,22 +128,36 @@ void serial_send(uint8_t data)
 /* detect edge of start bit */
 ISR(SERIAL_RXD_VECT)
 {
+    SERIAL_SOFT_DEBUG_TGL()
     SERIAL_RXD_INT_ENTER()
 
     uint8_t data = 0;
+
 #ifdef SERIAL_BIT_ORDER_MSB
     uint8_t mask = 0x80;
 #else
     uint8_t mask = 0x01;
 #endif
+
+#ifdef SERIAL_PARITY_ODD
+    uint8_t parity = 0;
+#elif defined(SERIAL_PARITY_EVEN)
+    uint8_t parity = 1;
+#endif
+
     /* to center of start bit */
     _delay_us(WAIT_US/2);
+    SERIAL_SOFT_DEBUG_TGL()
     do {
         /* to center of next bit */
         _delay_us(WAIT_US);
 
+    SERIAL_SOFT_DEBUG_TGL()
         if (SERIAL_RXD_READ()) {
             data |= mask;
+#if defined(SERIAL_PARITY_EVEN) || defined(SERIAL_PARITY_ODD)
+            parity ^= 1;
+#endif
         }
 #ifdef SERIAL_BIT_ORDER_MSB
         mask >>= 1;
@@ -126,14 +165,27 @@ ISR(SERIAL_RXD_VECT)
         mask <<= 1;
 #endif
     } while (mask);
+
+#if defined(SERIAL_PARITY_EVEN) || defined(SERIAL_PARITY_ODD)
+    /* to center of parity bit */
+    _delay_us(WAIT_US);
+    if (SERIAL_RXD_READ()) { parity ^= 1; }
+    SERIAL_SOFT_DEBUG_TGL()
+#endif
+
     /* to center of stop bit */
     _delay_us(WAIT_US);
 
     uint8_t next = (rbuf_head + 1) % RBUF_SIZE;
+#if defined(SERIAL_PARITY_EVEN) || defined(SERIAL_PARITY_ODD)
+    if (parity && next != rbuf_tail) {
+#else
     if (next != rbuf_tail) {
+#endif
         rbuf[rbuf_head] = data;
         rbuf_head = next;
     }
 
     SERIAL_RXD_INT_EXIT();
+    SERIAL_SOFT_DEBUG_TGL()
 }