Backlight control.

drivers/st7701/st7701.cpp

@@ -381,17 +381,11 @@ void __no_inline_not_in_flash_func(ST7701::fill_next_line()) {
     // if a backlight pin is provided then set it up for
     // pwm control
     if(lcd_bl != PIN_UNUSED) {
-      #if 0
       pwm_config cfg = pwm_get_default_config();
-      pwm_set_wrap(pwm_gpio_to_slice_num(lcd_bl), 65535);
+      pwm_config_set_wrap(&cfg, BACKLIGHT_PWM_TOP);
       pwm_init(pwm_gpio_to_slice_num(lcd_bl), &cfg, true);
       gpio_set_function(lcd_bl, GPIO_FUNC_PWM);
       set_backlight(0); // Turn backlight off initially to avoid nasty surprises
-      #else
-      gpio_init(lcd_bl);
-      gpio_set_dir(lcd_bl, GPIO_OUT);
-      gpio_put(lcd_bl, 0);
-      #endif
     }
 
     command(reg::SWRESET);
@@ -462,11 +456,7 @@ void __no_inline_not_in_flash_func(ST7701::fill_next_line()) {
     if(lcd_bl != PIN_UNUSED) {
       //update(); // Send the new buffer to the display to clear any previous content
       sleep_ms(50); // Wait for the update to apply
-      #if 0
       set_backlight(255); // Turn backlight on now surprises have passed
-      #else
-      gpio_put(lcd_bl, 1);
-      #endif
     }
   }
 
@@ -580,10 +570,11 @@ void __no_inline_not_in_flash_func(ST7701::fill_next_line()) {
   }
 
   void ST7701::set_backlight(uint8_t brightness) {
-    // gamma correct the provided 0-255 brightness value onto a
-    // 0-65535 range for the pwm counter
-    float gamma = 2.8;
-    uint16_t value = (uint16_t)(pow((float)(brightness) / 255.0f, gamma) * 65535.0f + 0.5f);
+    // At least on my hardware this gives reasonable control over the possible range of backlight brightness
+    uint16_t value;
+    if (brightness == 0) value = 0;
+    else if (brightness == 255) value = BACKLIGHT_PWM_TOP;
+    else value = 181 + (brightness * brightness) / 85;
     pwm_set_gpio_level(lcd_bl, value);
   }
 

drivers/st7701/st7701.hpp

@@ -47,6 +47,7 @@ namespace pimoroni {
     uint lcd_dot_clk = 22;
 
     static const uint32_t SPI_BAUD = 8'000'000;
+    static const uint32_t BACKLIGHT_PWM_TOP = 6200;
 
   public:
     // Parallel init

modules/c/presto/presto.c

@@ -5,12 +5,14 @@
 
 MP_DEFINE_CONST_FUN_OBJ_1(Presto___del___obj, Presto___del__);
 MP_DEFINE_CONST_FUN_OBJ_2(Presto_update_obj, Presto_update);
+MP_DEFINE_CONST_FUN_OBJ_2(Presto_set_backlight_obj, Presto_set_backlight);
 
 /***** Binding of Methods *****/
 
 static const mp_rom_map_elem_t Presto_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR___del__), MP_ROM_PTR(&Presto___del___obj) },
     { MP_ROM_QSTR(MP_QSTR_update), MP_ROM_PTR(&Presto_update_obj) },
+    { MP_ROM_QSTR(MP_QSTR_set_backlight), MP_ROM_PTR(&Presto_set_backlight_obj) },
 
     { MP_ROM_QSTR(MP_QSTR_WIDTH), MP_ROM_INT(WIDTH) },
     { MP_ROM_QSTR(MP_QSTR_HEIGHT), MP_ROM_INT(HEIGHT) },

modules/c/presto/presto.cpp

@@ -173,6 +173,18 @@ extern mp_obj_t Presto_update(mp_obj_t self_in, mp_obj_t graphics_in) {
     return mp_const_none;
 }
 
+mp_obj_t Presto_set_backlight(mp_obj_t self_in, mp_obj_t brightness) {
+    _Presto_obj_t *self = MP_OBJ_TO_PTR2(self_in, _Presto_obj_t);
+
+    float b = mp_obj_get_float(brightness);
+
+    if(b < 0 || b > 1.0f) mp_raise_ValueError("brightness out of range. Expected 0.0 to 1.0");
+
+    self->presto->set_backlight((uint8_t)(b * 255.0f));
+
+    return mp_const_none;
+}
+
 mp_obj_t Presto___del__(mp_obj_t self_in) {
     (void)self_in;
     //_Presto_obj_t *self = MP_OBJ_TO_PTR2(self_in, _Presto_obj_t);

modules/c/presto/presto.h

@@ -19,4 +19,5 @@ extern const mp_obj_type_t Presto_type;
 extern mp_obj_t Presto_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *all_args);
 extern mp_obj_t Presto_update(mp_obj_t self_in, mp_obj_t graphics_in);
 extern mp_int_t Presto_get_framebuffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, mp_uint_t flags);
+extern mp_obj_t Presto_set_backlight(mp_obj_t self_in, mp_obj_t brightness);
 extern mp_obj_t Presto___del__(mp_obj_t self_in);