README.MBX 2.6 KB

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  1. IMPORTANT NOTE - read before defining CONFIG_SYS_USE_OSCCLK in your board
  2. config file!!!
  3. WARNING: Wrong settings of this parameter have the potential to
  4. damage hardware by running the MBX's CPU at frequencies that exceed
  5. it's rating and/or overdriving the it's SPLL!
  6. Ramblings:
  7. 1) Motorola offered 12 different variants of the MBX, 6 823s and 6 860s.
  8. 2) Of these 12 variants, only 2 were entry level boards.
  9. 3) I believe that the 2 entry level boards were the only ones that
  10. used OSCM clocking. I can't be completely certain of this at this
  11. point.
  12. 4) Motorola never offered an MBX that ran faster than 50Mhz.
  13. 5) The 10, non-entry level boards, ran at 40Mhz.
  14. 6) The EXTCLK input has a minimum clock of 15Mhz for the 823/860.
  15. 7) Motorola no longer sells MBXs.
  16. Based on this information, I can surmise that the default power-on
  17. reset clocking was one of the following three options.
  18. Multiplier SPLL Options
  19. ------------------------------------
  20. 513 OSCM is SPLL input
  21. 5 OSCM is SPLL input
  22. 1 EXTCLK is SPLL input
  23. The forth option:
  24. 5 EXTCLK is SPLL input
  25. is not possible on MBXs. This is because the minimum EXTCLK input
  26. frequency is 15Mhz. 5 * 15Mhz = 75 Mhz. There was no variant that ran
  27. above 50 Mhz.
  28. The board I have borrowed definitely uses a multiplier of 1 for
  29. EXTCLK and runs at 40Mhz. I even went so far as to put a scope on it.
  30. One of the two default OSCM modes are most likely what was used on
  31. the entry level boards to cheapen them by eliminating the external
  32. crystal oscillator.
  33. To add insult to injury, the stupid 860 PLPRCR register retains it's
  34. multiplication factor through hard resets. You can't clear it out
  35. because it is battery backed and once it is set wrong, it stays
  36. wrong. The only way to reset it, so that it takes on it's default
  37. multiplier is to disconnect all power including external, batteries,
  38. as well discharging caps on the board. This precludes the fact that
  39. your 860 may be quite DEAD by this time!
  40. If you don't setup the multiplication factor for boards that use the
  41. OSCM input, they won't run correctly, but at least they won't be
  42. dead.
  43. Addtionally, there is no good way to determine the clock input source
  44. from CPU register data. The only way to deal with this is either hard
  45. code it, determine the correct value with some rather NASTY timing
  46. loops, or try to grok it from external data sources. Motorola
  47. firmware opts for the NASTY timing loops, but needs to configure the
  48. serial ports to do so.
  49. You may have a legitimate need to define CONFIG_SYS_USE_OSCCLK if your
  50. MBX8xx board is using the OSCM clocking mode.
  51. You better know what you are doing here.