+ *
+ * The Divider for PTA (refresh timer) configuration is based on an
+ * example SDRAM configuration (64 MBit, one bank). The adjustment to
+ * the number of chip selects (NCS) and the actually needed refresh
+ * rate is done by setting MPTPR.
+ *
+ * PTA is calculated from
+ * PTA = (gclk * Trefresh) / ((2 ^ (2 * DFBRG)) * PTP * NCS)
+ *
+ * gclk CPU clock (not bus clock!)
+ * Trefresh Refresh cycle * 4 (four word bursts used)
+ *
+ * 4096 Rows from SDRAM example configuration
+ * 1000 factor s -> ms
+ * 32 PTP (pre-divider from MPTPR) from SDRAM example configuration
+ * 4 Number of refresh cycles per period
+ * 64 Refresh cycle in ms per number of rows
+ * --------------------------------------------
+ * Divider = 4096 * 32 * 1000 / (4 * 64) = 512000
+ *
+ * 50 MHz => 50.000.000 / Divider = 98
+ * 66 Mhz => 66.000.000 / Divider = 129
+ * 80 Mhz => 80.000.000 / Divider = 156