- * Memory Periodic Timer Prescaler
- * Periodic timer for refresh, start with refresh rate for 40 MHz clock
- * (CFG_8xx_CPUCLK_MIN / CFG_PTA_PER_CLK)
+ * Periodic timer (MAMR[PTx]) for 4 * 7.8 us refresh (= 31.2 us per quad)
+ *
+ * CPUclock(MHz) * 31.2
+ * CFG_MAMR_PTA = ----------------------------------- with DFBRG = 0
+ * 2^(2*SCCR[DFBRG]) * MPTPR_PTP_DIV16
+ *
+ * CPU clock = 15 MHz: CFG_MAMR_PTA = 29 -> 4 * 7.73 us
+ * CPU clock = 50 MHz: CFG_MAMR_PTA = 97 -> 4 * 7.76 us
+ * CPU clock = 66 MHz: CFG_MAMR_PTA = 128 -> 4 * 7.75 us
+ * CPU clock = 133 MHz: CFG_MAMR_PTA = 255 -> 4 * 7.67 us
+ *
+ * Value 97 is for 4 * 7.8 us at 50 MHz. So the refresh cycle requirement will
+ * be met also in the default configuration, i.e. if environment variable
+ * 'cpuclk' is not set.