From 2bf31a31da336b628825aced248396b8ba3e3105 Mon Sep 17 00:00:00 2001
From: rob-ng15 <58272847+rob-ng15@users.noreply.github.com>
Date: Sat, 21 Mar 2020 07:37:21 +0000
Subject: [PATCH] Reclock spi sdcard access after initialisation

Depends upon https://github.com/enjoy-digital/litex/pull/435

After initialising the card, reclock the card, aiming for ~16MHz (divider is rounded up, as slower speed is safer), but a maximum of half of the processor speed.

Tested with the card being clocked to 12.5MHz on de10nano
---
 litex/soc/software/libbase/spisdcard.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/litex/soc/software/libbase/spisdcard.c b/litex/soc/software/libbase/spisdcard.c
index 635809070..fd5b327ff 100644
--- a/litex/soc/software/libbase/spisdcard.c
+++ b/litex/soc/software/libbase/spisdcard.c
@@ -444,6 +444,17 @@ uint8_t spi_sdcard_readMBR(void)
         return FAILURE;
     }
 
+    // Reclock the card
+    // Calculate 16MHz as an integer divider from the CONFIG_CLOCK_FREQUENCY
+    // Add 1 as will be rounded down
+    // Always ensure divider is at least 2 - half the processor speed
+    int divider;
+    divider = (int)(CONFIG_CLOCK_FREQUENCY/(16e6)) + 1;
+    if( divider<2 )
+        divider=2;
+    printf("Reclocking from %dKHz to %dKHz\n\n", CONFIG_CLOCK_FREQUENCY/(int)spisdcard_clk_divider_read()/1000, CONFIG_CLOCK_FREQUENCY/divider/1000);
+    spisdcard_clk_divider_write(divider);
+    
     // Read in FAT16 File Allocation Table, array of 16bit unsinged integers
     // Calculate Storage from TOP of MAIN RAM
     sdCardFatTable = (uint16_t *)(MAIN_RAM_BASE+MAIN_RAM_SIZE-sdCardFatBootSector.sector_size*sdCardFatBootSector.fat_size_sectors);