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diff --git a/doc/README.MBX b/doc/README.MBX new file mode 100644 index 0000000000..679228ec39 --- /dev/null +++ b/doc/README.MBX @@ -0,0 +1,68 @@ +IMPORTANT NOTE - read before defining CFG_USE_OSCCLK in your board + config file!!! + + +WARNING: Wrong settings of this parameter have the potential to +damage hardware by running the MBX's CPU at frequencies that exceed +it's rating and/or overdriving the it's SPLL! + + +Ramblings: +1) Motorola offered 12 different variants of the MBX, 6 823s and 6 860s. +2) Of these 12 variants, only 2 were entry level boards. +3) I believe that the 2 entry level boards were the only ones that + used OSCM clocking. I can't be completely certain of this at this + point. +4) Motorola never offered an MBX that ran faster than 50Mhz. +5) The 10, non-entry level boards, ran at 40Mhz. +6) The EXTCLK input has a minimum clock of 15Mhz for the 823/860. +7) Motorola no longer sells MBXs. + +Based on this information, I can surmise that the default power-on +reset clocking was one of the following three options. + +Multiplier SPLL Options +------------------------------------ +513 OSCM is SPLL input +5 OSCM is SPLL input +1 EXTCLK is SPLL input + +The forth option: + +5 EXTCLK is SPLL input + +is not possible on MBXs. This is because the minimum EXTCLK input +frequency is 15Mhz. 5 * 15Mhz = 75 Mhz. There was no variant that ran +above 50 Mhz. + +The board I have borrowed definitely uses a multiplier of 1 for +EXTCLK and runs at 40Mhz. I even went so far as to put a scope on it. + +One of the two default OSCM modes are most likely what was used on +the entry level boards to cheapen them by eliminating the external +crystal oscillator. + +To add insult to injury, the stupid 860 PLPRCR register retains it's +multiplication factor through hard resets. You can't clear it out +because it is battery backed and once it is set wrong, it stays +wrong. The only way to reset it, so that it takes on it's default +multiplier is to disconnect all power including external, batteries, +as well discharging caps on the board. This precludes the fact that +your 860 may be quite DEAD by this time! + +If you don't setup the multiplication factor for boards that use the +OSCM input, they won't run correctly, but at least they won't be +dead. + +Addtionally, there is no good way to determine the clock input source +from CPU register data. The only way to deal with this is either hard +code it, determine the correct value with some rather NASTY timing +loops, or try to grok it from external data sources. Motorola +firmware opts for the NASTY timing loops, but needs to configure the +serial ports to do so. + + +You may have a legitimate need to define CFG_USE_OSCCLK if your +MBX8xx board is using the OSCM clocking mode. + +You better know what you are doing here. |