Add USB CDC and loop-back test

stm32f103c8t6/src/usb_util.c

@@ -0,0 +1,38 @@
+#include "usb_util.h"
+
+void USB_PMAToMemory(void *mem, uint16_t offset, size_t length)
+{
+    uint8_t *dst = (uint8_t*)mem;
+    uint8_t *pma = (uint8_t*)(USB_PMA_ADDR + 2 * offset);
+
+    for(unsigned int i = 0; i < length; i++)
+    {
+        *dst++ = *pma++;
+        if(i & 1)
+        {
+            pma += 2;
+        }
+    }
+}
+
+void USB_MemoryToPMA(uint16_t offset, const void *mem, size_t length)
+{
+    // Only words can be copied. Thus, if the length is not even, it has to be
+    // incremented to ensure that the last byte is copied. Since the PMA buffer
+    // always has even size, this is not a problem.
+    if(length & 1)
+    {
+        length++;
+    }
+
+    const uint8_t *src = (const uint8_t*)mem;
+
+    uint16_t *pma = (uint16_t*)(USB_PMA_ADDR + 2 * offset);
+    for(unsigned int i = 0; i < length / 2; i++)
+    {
+        uint16_t tmp = src[2 * i] | (src[2 * i + 1] << 8);
+        *pma++ = tmp;
+        pma++;
+    }
+}
+