usb: add nack to protocol

2 files changed, 4 insertions, 2 deletions

diff --git a/doc/protocol b/doc/protocol
index 2273f40..5d2cb65 100644
--- a/doc/protocol
+++ b/doc/protocol
@@ -1,5 +1,5 @@
 <command> ::= <opcode> " " <args> "\n"
-<response> ::= <retval> "\n"
+<response> ::= (<retval>|"nack") "\n"
 
 Echo:
 <opcode> ::= "e"
diff --git a/fw/usb.c b/fw/usb.c
index 7f283e1..532bafd 100644
--- a/fw/usb.c
+++ b/fw/usb.c
@@ -86,8 +86,10 @@ idleState(void) {
 		case 'e': state.next = echoState; break;
 		case 'w': state.next = writeEepromState; break;
 		case 'r': state.next = readEepromState; break;
-		default: break; // invalid command
+		default: putsUSBUSART("nack\n"); break; // invalid command
 		}
+	} else if (readLen > 0) {
+		putsUSBUSART("nack\n");
 	}
 
 	return &state;