add a 0x00 command that means "I have nothing to say"

diff --git a/main.c b/main.c
index 82739b0..9c368d5 100644 (file)
--- a/main.c
+++ b/main.c
@@ -52,11 +52,13 @@ static uint16_t icp_prev;
 /*
  * SPI protocol:
  *
- * A command is stored on 2 bytes. The first one has its msb to 0, and the
- * second one to 1. The first received byte contains the command number, and the
- * msb of the servo value. The second byte contains the lsb of the servo value.
+ * A command is stored on 2 bytes (except command 0). The first byte
+ * has its most significant bit to 0, and the second one to 1. The
+ * first received byte contains the command number, and the msb of the
+ * servo value. The second byte contains the lsb of the servo value.
  *
- * Commands 0 to NB_SERVO are to set the value of servo.
+ * Command 0 is only one byte long, it means "I have nothing to say".
+ * Commands 1 to NB_SERVO+1 are to set the value of servo.
  * Command 14 is to enable bypass mode.
  * Command 15 is to disable bypass mode.
  */
@@ -106,6 +108,10 @@ static void poll_spi(void)
 
        if (rxidx == 0) {
                byte0.u8 = c;
+
+               /* command num 0 is ignored */
+               if (byte0.cmd_num == 0)
+                       return;
        }
        else {
                uint16_t val;
@@ -114,10 +120,10 @@ static void poll_spi(void)
 
                /* process command */
 
-               if (byte0.cmd_num < NB_SERVO) {
+               if (byte0.cmd_num < NB_SERVO+1) {
                        val = (uint16_t)byte0.val_msb << 7;
                        val += byte1.val_lsb;
-                       servo_table[byte0.cmd_num].command = val;
+                       servo_table[byte0.cmd_num-1].command = val;
                }
                else if (byte0.cmd_num == BYPASS_ENABLE) {
                        bypass = 1;