mtm_computer: make board.DAC() an actual singleton

Author: todbot

ports/raspberrypi/boards/mtm_computer/board.c

@@ -12,16 +12,22 @@
 
 // board.DAC() — factory function that constructs an mcp4822.MCP4822 with
 // the MTM Workshop Computer's DAC pins (GP18=SCK, GP19=SDI, GP21=CS).
+
+static mp_obj_t board_dac_singleton = MP_OBJ_NULL;
+
 static mp_obj_t board_dac_factory(void) {
-    mcp4822_mcp4822_obj_t *dac = mp_obj_malloc_with_finaliser(
-        mcp4822_mcp4822_obj_t, &mcp4822_mcp4822_type);
-    common_hal_mcp4822_mcp4822_construct(
-        dac,
-        &pin_GPIO18,   // clock (SCK)
-        &pin_GPIO19,   // mosi (SDI)
-        &pin_GPIO21,   // cs
-        1);            // gain 1x
-    return MP_OBJ_FROM_PTR(dac);
+    if (board_dac_singleton == MP_OBJ_NULL) {
+        mcp4822_mcp4822_obj_t *dac = mp_obj_malloc_with_finaliser(
+            mcp4822_mcp4822_obj_t, &mcp4822_mcp4822_type);
+        common_hal_mcp4822_mcp4822_construct(
+            dac,
+            &pin_GPIO18,   // clock (SCK)
+            &pin_GPIO19,   // mosi (SDI)
+            &pin_GPIO21,   // cs
+            1);            // gain 1x
+        board_dac_singleton = MP_OBJ_FROM_PTR(dac);
+    }
+    return board_dac_singleton;
 }
 MP_DEFINE_CONST_FUN_OBJ_0(board_dac_obj, board_dac_factory);