| 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. |