stm32/sdram: Make SDRAM refresh count configurable by a board.

Refresh count calculations were using a hard-coded SDRAM frequency and
refresh cycles, so change them to values that can be set by a board.

And set these options to their existing values on STM32F769DISC and
STM32F7DISC boards.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

Author: iabdalkader

ports/stm32/boards/STM32F769DISC/mpconfigboard.h

@@ -141,6 +141,7 @@ extern struct _spi_bdev_t spi_bdev;
 
 #define MICROPY_HW_SDRAM_BURST_LENGTH       1
 #define MICROPY_HW_SDRAM_CAS_LATENCY        2
+#define MICROPY_HW_SDRAM_FREQUENCY_KHZ      (90000) // 90 MHz
 #define MICROPY_HW_SDRAM_COLUMN_BITS_NUM    8
 #define MICROPY_HW_SDRAM_ROW_BITS_NUM       12
 #define MICROPY_HW_SDRAM_MEM_BUS_WIDTH      32
@@ -150,6 +151,7 @@ extern struct _spi_bdev_t spi_bdev;
 #define MICROPY_HW_SDRAM_RBURST             (1)
 #define MICROPY_HW_SDRAM_WRITE_PROTECTION   (0)
 #define MICROPY_HW_SDRAM_AUTOREFRESH_NUM    (8)
+#define MICROPY_HW_SDRAM_REFRESH_CYCLES     8192
 
 // See pins.csv for CPU pin mapping
 #define MICROPY_HW_FMC_SDCKE0  (pyb_pin_FMC_SDCKE0)

ports/stm32/boards/STM32F7DISC/mpconfigboard.h

@@ -111,6 +111,7 @@ void STM32F7DISC_board_early_init(void);
 
 #define MICROPY_HW_SDRAM_BURST_LENGTH       1
 #define MICROPY_HW_SDRAM_CAS_LATENCY        2
+#define MICROPY_HW_SDRAM_FREQUENCY_KHZ      (90000) // 90 MHz
 #define MICROPY_HW_SDRAM_COLUMN_BITS_NUM    8
 #define MICROPY_HW_SDRAM_ROW_BITS_NUM       12
 #define MICROPY_HW_SDRAM_MEM_BUS_WIDTH      16
@@ -120,6 +121,7 @@ void STM32F7DISC_board_early_init(void);
 #define MICROPY_HW_SDRAM_RBURST             (1)
 #define MICROPY_HW_SDRAM_WRITE_PROTECTION   (0)
 #define MICROPY_HW_SDRAM_AUTOREFRESH_NUM    (8)
+#define MICROPY_HW_SDRAM_REFRESH_CYCLES     8192
 
 #define MICROPY_HW_FMC_SDCKE0   (pin_C3)
 #define MICROPY_HW_FMC_SDNE0    (pin_H3)

ports/stm32/sdram.c

@@ -245,15 +245,15 @@ static void sdram_init_seq(SDRAM_HandleTypeDef
     /* Send the command */
     HAL_SDRAM_SendCommand(hsdram, command, 0x1000);
 
-    /* Step 8: Set the refresh rate counter
+    /* Step 8: Set the refresh rate counter.
+       Assuming 90MHz frequency, 8192 refresh cycles and 64ms refresh rate:
        RefreshRate = 64 ms / 8192 cyc = 7.8125 us/cyc
-
        RefreshCycles = 7.8125 us * 90 MHz = 703
        According to the formula on p.1665 of the reference manual,
        we also need to subtract 20 from the value, so the target
        refresh rate is 703 - 20 = 683.
      */
-    #define REFRESH_COUNT (MICROPY_HW_SDRAM_REFRESH_RATE * 90000 / 8192 - 20)
+    #define REFRESH_COUNT (MICROPY_HW_SDRAM_REFRESH_RATE * MICROPY_HW_SDRAM_FREQUENCY_KHZ / MICROPY_HW_SDRAM_REFRESH_CYCLES - 20)
     HAL_SDRAM_ProgramRefreshRate(hsdram, REFRESH_COUNT);
 
     #if defined(STM32F7) || defined(STM32H7)