From 5b27cf9c79fa22544e3aa98c702f0496a7e2a6e7 Mon Sep 17 00:00:00 2001
From: XANTRONIX Development <dev@xantronix.com>
Date: Tue, 7 Jun 2016 17:59:23 -0500
Subject: [PATCH] Refactor print receiver to calculate checksums and raise CRC
 error when necessary

---
 avr/recv.c | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/avr/recv.c b/avr/recv.c
index da9fc95..924d03d 100644
--- a/avr/recv.c
+++ b/avr/recv.c
@@ -9,7 +9,14 @@
 
 static volatile uint16_t offset_header = 0,
                          offset_body   = 0,
-                         size_body     = 0;
+                         size_body     = 0,
+                         sum_calc      = 0,
+                         sum_footer    = 0;
+
+static volatile tabby_printer_response response = {
+    .device = TABBY_PRINTER_DEVICE_ID,
+    .status = TABBY_PRINTER_OK
+};
 
 /*
  * SPI byte receipt interrupt vector
@@ -25,6 +32,9 @@ ISR(SPI_STC_vect) {
             if (in == TABBY_PRINTER_SYNC_1) {
                 offset_header++;
                 offset_body = 0;
+                sum_calc    = 0;
+
+                response.status = TABBY_PRINTER_OK;
             }
 
             break;
@@ -43,6 +53,7 @@ ISR(SPI_STC_vect) {
         case 2:
         case 3: {
             offset_header++;
+            sum_calc += in;
 
             break;
         }
@@ -50,6 +61,7 @@ ISR(SPI_STC_vect) {
         case 4: {
             size_body = in;
 
+            sum_calc += in;
             offset_header++;
 
             break;
@@ -64,20 +76,32 @@ ISR(SPI_STC_vect) {
                 offset_header++;
             }
 
+            sum_calc += in;
+
             break;
         }
 
         default: {
-            if (offset_body <= size_body) {
+            if (offset_body < size_body) {
                 offset_body++;
+                sum_calc += in;
+            } else if (offset_body == size_body) {
+                offset_body++;
+                sum_footer = in;
             } else if (offset_body == size_body + 1) {
                 offset_body++;
 
-                out = TABBY_PRINTER_DEVICE_ID;
+                sum_footer |= in << 8;
+
+                if (sum_footer != sum_calc) {
+                    response.status |= TABBY_PRINTER_SUM;
+                }
+
+                out = response.device;
             } else if (offset_body == size_body + 2) {
                 offset_body++;
 
-                out = TABBY_PRINTER_OK;
+                out = response.status;
             } else if (offset_body == size_body + 3) {
                 offset_header = 0;
             }