From: Jan "Yenya" Kasprzak <kas@fi.muni.cz>
Date: Sat, 23 May 2015 19:02:37 +0000 (+0200)
Subject: Do more RX checks in the IRQ handler.
X-Git-Url: https://www.fi.muni.cz/~kas/git//home/kas/public_html/git/?p=openparking.git;a=commitdiff_plain;h=63da2000b15179f9c6c42965f9cc0115ed95c5f7

Do more RX checks in the IRQ handler.
---

diff --git a/firmware/modbus.c b/firmware/modbus.c
index 00a9370..e45f5f0 100644
--- a/firmware/modbus.c
+++ b/firmware/modbus.c
@@ -169,18 +169,10 @@ uint8_t modbus_poll()
 		if (transmitting)
 			return 0;
 
-		if (buf_len == 0) // nothing received yet
-			return 0;
-
 		if (get_clock() - last_rx < REQ_TIMEOUT) // still receiving
 			return 0;
 
-		if (buf_len < 4) { // too short
-			buf_len = 0;
-			return 0;
-		}
-
-		if (buffer[0] != mb_unit_id) { // not for myself
+		if (buf_len < 4) { // too short (or not for us)
 			buf_len = 0;
 			return 0;
 		}
@@ -241,16 +233,28 @@ uint8_t modbus_poll()
 ISR(USART_RX_vect)
 {
 	uint8_t rx_byte = UDR0;
+	clock_t now = get_clock();
+
+	if (transmitting) // how did we get here? discard it
+		goto out;
 
-	if (transmitting) // discard it
-		return;
+	if (buf_len && buffer[0] != mb_unit_id) // not for us
+		goto out;
 
-	buffer[buf_len] = rx_byte;
+	if (buf_len == BUFSIZE) { // overrun - discard the packet
+		buffer[0] = 0xFF;
+		buf_len = 1;
+		goto out;
+	}
 
-	if (buf_len + 1 < BUFSIZE) // ignore overruns
-		buf_len++;
+	if (now - last_rx >= REQ_TIMEOUT) { // new packet; start over
+		buf_len = 0;
+	}
 
-	last_rx = get_clock();
+	// TODO: we can probably calculate the CRC here as well
+	buffer[buf_len++] = rx_byte;
+out:
+	last_rx = now;
 }
 
 ISR(USART_TX_vect)