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