Add 800x480p30 mode

this requires less OC than 800x480p60 and is verified to work with
https://www.adafruit.com/product/2232

examples/1bit_text/1bit_text.ino

@@ -18,6 +18,10 @@ DVItext1 display(DVI_RES_640x240p60, adafruit_feather_dvi_cfg);
 // Here's how a 100x60 char display might be declared:
 //DVItext1 display(DVI_RES_800x480p60, adafruit_feather_dvi_cfg);
 
+// A reduced refresh rate display doesn't as aggressive an over-clock
+// This timing is verified to work on https://www.adafruit.com/product/2232
+//DVItext1 display(DVI_RES_800x240p30, adafruit_feather_dvi_cfg);
+
 void setup() { // Runs once on startup
   if (!display.begin()) { // Blink LED if insufficient RAM
     pinMode(LED_BUILTIN, OUTPUT);

src/PicoDVI.cpp

@@ -23,10 +23,13 @@ static struct {
 } dvispec[] = {
     {dvi_timing_640x480p_60hz, VREG_VOLTAGE_1_20, 320, 240, 2},
     {dvi_timing_800x480p_60hz, VREG_VOLTAGE_1_20, 400, 240, 2},
+    {dvi_timing_800x480p_30hz, VREG_VOLTAGE_1_20, 400, 240, 2},
     {dvi_timing_640x480p_60hz, VREG_VOLTAGE_1_20, 640, 480, 1},
     {dvi_timing_800x480p_60hz, VREG_VOLTAGE_1_20, 800, 480, 1},
+    {dvi_timing_800x480p_30hz, VREG_VOLTAGE_1_20, 800, 480, 1},
     {dvi_timing_640x480p_60hz, VREG_VOLTAGE_1_20, 640, 240, 2},
     {dvi_timing_800x480p_60hz, VREG_VOLTAGE_1_20, 800, 240, 2},
+    {dvi_timing_800x480p_30hz, VREG_VOLTAGE_1_20, 800, 240, 2},
     // Additional resolutions might get added here if the overclock issue can
     // be sorted out. Regardless, always keep this list 1:1 in sync with the
     // DVIresolution enum in PicoDVI.h.

src/PicoDVI.h

@@ -21,10 +21,13 @@
 enum DVIresolution {
   DVI_RES_320x240p60 = 0,
   DVI_RES_400x240p60,
+  DVI_RES_400x240p30, // Reduced refresh rate, less overclock required
   DVI_RES_640x480p60,
   DVI_RES_800x480p60,
+  DVI_RES_800x480p30, // Reduced refresh rate, less overclock required
   DVI_RES_640x240p60, // "Tall" pixels, e.g. for 80-column text mode
   DVI_RES_800x240p60, // Same, 100-column
+  DVI_RES_800x240p30, // Reduced refresh rate, less overclock required
   DVI_RES_1280x720p30 // Experimenting, not working, plz don't use
 };
 

src/libdvi/dvi_timing.c

@@ -65,6 +65,26 @@ const struct dvi_timing __dvi_const(dvi_timing_800x480p_60hz) = {
 	.bit_clk_khz     = 295200
 };
 
+// 800x480p 30 Hz, 148 MHz bit clock. This is not a CEA mode either , but needs
+// lower overclock compared to dvi_timing_800x480p_60hz and the timing is
+// verified to work on https://www.adafruit.com/product/2232
+const struct dvi_timing __dvi_const(dvi_timing_800x480p_30hz) = {
+        .h_sync_polarity = false,
+        .h_front_porch   = 24,
+        .h_sync_width    = 72,
+        .h_back_porch    = 96,
+        .h_active_pixels = 800,
+
+        .v_sync_polarity = true,
+        .v_front_porch   = 3,
+        .v_sync_width    = 10,
+        .v_back_porch    = 4,
+        .v_active_lines  = 480,
+
+        .bit_clk_khz     = 147600
+};
+
+
 // SVGA reduced blanking (355 MHz bit clock) -- valid CVT mode, less common
 // than fully-blanked SVGA, but doesn't require such a high system clock
 const struct dvi_timing __dvi_const(dvi_timing_800x600p_reduced_60hz) = {

src/libdvi/dvi_timing.h

@@ -75,6 +75,7 @@ extern const uint32_t dvi_ctrl_syms[4];
 
 extern const struct dvi_timing dvi_timing_640x480p_60hz;
 extern const struct dvi_timing dvi_timing_800x480p_60hz;
+extern const struct dvi_timing dvi_timing_800x480p_30hz;
 extern const struct dvi_timing dvi_timing_800x600p_60hz;
 extern const struct dvi_timing dvi_timing_960x540p_60hz;
 extern const struct dvi_timing dvi_timing_1280x720p_30hz;