wrote the code for the BBB side
diff --git a/bbb_cape/src/bbb/crc.cc b/bbb_cape/src/bbb/crc.cc
new file mode 100644
index 0000000..b770f37
--- /dev/null
+++ b/bbb_cape/src/bbb/crc.cc
@@ -0,0 +1,42 @@
+#include "bbb/crc.h"
+
+#include "aos/common/once.h"
+
+// There are various implementations that look a lot like this scattered around
+// the internet.
+
+namespace cape {
+namespace {
+
+const uint32_t kPolynomial = 0x04c11db7;
+
+const uint32_t *GenerateTable() {
+ static uint32_t table[256];
+
+ for (int i = 0; i < 256; ++i) {
+ uint32_t c = i << 24;
+ for (int j = 8; j > 0; --j) {
+ c = c & 0x80000000 ? ((c << 1) ^ kPolynomial) : (c << 1);
+ }
+ table[i] = c;
+ }
+
+ return table;
+}
+
+} // namespace
+
+uint32_t CalculateChecksum(uint8_t *data, size_t length) {
+ static ::aos::once<const uint32_t> table_once(GenerateTable);
+ const uint32_t *const table = table_once.Get();
+
+ uint32_t r = 0xFFFFFFFF;
+
+ for (size_t i = 0; i < length; ++i) {
+ r = (r << 8) ^ table[(r >> 24) ^ data[i]];
+ }
+
+ return ~r;
+}
+
+} // namespace cape