Cite ST BSP for SDRAM AC timings; add constants on F429/F469/F769

F429/F469/F769 previously had a single SDRAM_Min_Delay_In_ns; replace
with the same SDRAM_TRAS_In_Ns / SDRAM_TRC_In_Ns / SDRAM_TXSR_In_Ns
trio that the shared SDRAM driver expects. F746 comment block reworked
to cite the actual on-board part (Micron MT48LC4M32B2B5-6A) and the ST
BSP source; tXSR corrected from 70 to 67 to match the Micron datasheet.

Author: pat-rogers

boards/stm32_common/stm32f429disco/stm32-board.ads

@@ -132,7 +132,12 @@ package STM32.Board is
    SDRAM_Read_Pipe       : constant STM32.FMC.FMC_SDRAM_Read_Pipe_Delay :=
                              STM32.FMC.FMC_ReadPipe_Delay_1;
    SDRAM_Refresh_Cnt     : constant := 1386;
-   SDRAM_Min_Delay_In_ns : constant := 70;
+
+   --  SDRAM AC timings (ns) for the on-board ISSI IS42S16400J
+   --  Values from the ST BSP in stm32f429i_discovery_sdram.c at SDCLK = 90 MHz:
+   SDRAM_TRAS_In_Ns      : constant := 42;  --  TRAS  
+   SDRAM_TRC_In_Ns       : constant := 70;  --  TRC   
+   SDRAM_TXSR_In_Ns      : constant := 70;  --  TXSR
 
    ---------------
    -- SPI5 Pins --

boards/stm32_common/stm32f469disco/stm32-board.ads

@@ -129,7 +129,11 @@ package STM32.Board is
    SDRAM_Read_Pipe       : constant STM32.FMC.FMC_SDRAM_Read_Pipe_Delay :=
                              STM32.FMC.FMC_ReadPipe_Delay_0;
    SDRAM_Refresh_Cnt     : constant := 16#0569#;
-   SDRAM_Min_Delay_In_ns : constant := 70;
+   --  SDRAM AC timings (ns) for the on-board Micron MT48LC4M32B2B5
+   --  Values from the ST BSP in stm32469i_discovery_sdram.c at SDCLK = 90 MHz:
+   SDRAM_TRAS_In_Ns      : constant := 42;  --  TRAS
+   SDRAM_TRC_In_Ns       : constant := 70;  --  TRC
+   SDRAM_TXSR_In_Ns      : constant := 70;  --  TXSR
 
    ---------
    -- I2C --

boards/stm32_common/stm32f746disco/stm32-board.ads

@@ -116,9 +116,11 @@ package STM32.Board is
    SDRAM_Read_Pipe       : constant STM32.FMC.FMC_SDRAM_Read_Pipe_Delay :=
                              STM32.FMC.FMC_ReadPipe_Delay_0;
    SDRAM_Refresh_Cnt     : constant := 1539;
-   SDRAM_TRAS_In_Ns      : constant := 42;  --  IS42S32400F min self-refresh time
-   SDRAM_TRC_In_Ns       : constant := 60;  --  IS42S32400F min row cycle time
-   SDRAM_TXSR_In_Ns      : constant := 70;  --  IS42S32400F min exit self-refresh delay
+   --  SDRAM AC timings (ns) for the on-board Micron MT48LC4M32B2B5-6A
+   --  Values from the ST BSP in stm32746g_discovery_sdram.c at SDCLK = 100 MHz:
+   SDRAM_TRAS_In_Ns      : constant := 42;  --  TRAS
+   SDRAM_TRC_In_Ns       : constant := 60;  --  TRC
+   SDRAM_TXSR_In_Ns      : constant := 67;  --  TXSR
 
    ---------
    -- I2C --

boards/stm32_common/stm32f769disco/stm32-board.ads

@@ -125,7 +125,11 @@ package STM32.Board is
    SDRAM_Read_Pipe       : constant STM32.FMC.FMC_SDRAM_Read_Pipe_Delay :=
                              STM32.FMC.FMC_ReadPipe_Delay_0;
    SDRAM_Refresh_Cnt     : constant := 16#0603#;
-   SDRAM_Min_Delay_In_ns : constant := 60;
+   --  SDRAM AC timings (ns) for the on-board Micron MT48LC4M32B2B5-6A
+   --  Values from the ST BSP in stm32f769i_discovery_sdram.c at SDCLK = 100 MHz:
+   SDRAM_TRAS_In_Ns      : constant := 42;  --  TRAS
+   SDRAM_TRC_In_Ns       : constant := 60;  --  TRC
+   SDRAM_TXSR_In_Ns      : constant := 67;  --  TXSR
 
    ---------
    -- I2C --