Try direct output to HT1632C.

src/board.c

@@ -377,13 +377,33 @@ void display_WriteData(const uint8_t * data, uint8_t addr, uint8_t len) {
   }
 
 #ifdef FRAMEBUFFER_ROTATE
+  int i;
+
+  GPIO_SPI_SW();
+  HT1632C_CS_ON;
+  i = (0x280 | addr);
+  // write addr
+  _display_WriteBits(i, 0x200);
+
+  GPIO_SPI_HW();
+  while ((SPI1->SR & SPI_SR_BSY) != 0) { __NOP(); }
+  for (i=0; i<len; i++) {
+    while ((SPI1->SR & SPI_SR_TXE) == 0) { __NOP(); }
+    SPI1->DR = data[i];
+  }
+  // wait for end of transmission
+  while ((SPI1->SR & SPI_SR_BSY) != 0) { __NOP(); }
+  HT1632C_CS_OFF;
+
   /* Copy given data to framebuffer */
+  /*
   int i, a;
   for (i=0; i<len; i++) {
     a = addr + i;
     display_Buffer[a] = data[i];
   }
   //display_WriteBuffer();
+  */
 #else
   uint16_t a = 0x280 | addr;