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wdenk865f3f52001-12-27 23:05:33 +00001IMPORTANT NOTE - read before defining CFG_USE_OSCCLK in your board
2 config file!!!
3
4
5WARNING: Wrong settings of this parameter have the potential to
6damage hardware by running the MBX's CPU at frequencies that exceed
7it's rating and/or overdriving the it's SPLL!
8
9
10Ramblings:
111) Motorola offered 12 different variants of the MBX, 6 823s and 6 860s.
122) Of these 12 variants, only 2 were entry level boards.
133) I believe that the 2 entry level boards were the only ones that
14 used OSCM clocking. I can't be completely certain of this at this
15 point.
164) Motorola never offered an MBX that ran faster than 50Mhz.
175) The 10, non-entry level boards, ran at 40Mhz.
186) The EXTCLK input has a minimum clock of 15Mhz for the 823/860.
197) Motorola no longer sells MBXs.
20
21Based on this information, I can surmise that the default power-on
22reset clocking was one of the following three options.
23
24Multiplier SPLL Options
25------------------------------------
26513 OSCM is SPLL input
275 OSCM is SPLL input
281 EXTCLK is SPLL input
29
30The forth option:
31
325 EXTCLK is SPLL input
33
34is not possible on MBXs. This is because the minimum EXTCLK input
35frequency is 15Mhz. 5 * 15Mhz = 75 Mhz. There was no variant that ran
36above 50 Mhz.
37
38The board I have borrowed definitely uses a multiplier of 1 for
39EXTCLK and runs at 40Mhz. I even went so far as to put a scope on it.
40
41One of the two default OSCM modes are most likely what was used on
42the entry level boards to cheapen them by eliminating the external
43crystal oscillator.
44
45To add insult to injury, the stupid 860 PLPRCR register retains it's
46multiplication factor through hard resets. You can't clear it out
47because it is battery backed and once it is set wrong, it stays
48wrong. The only way to reset it, so that it takes on it's default
49multiplier is to disconnect all power including external, batteries,
50as well discharging caps on the board. This precludes the fact that
51your 860 may be quite DEAD by this time!
52
53If you don't setup the multiplication factor for boards that use the
54OSCM input, they won't run correctly, but at least they won't be
55dead.
56
57Addtionally, there is no good way to determine the clock input source
58from CPU register data. The only way to deal with this is either hard
59code it, determine the correct value with some rather NASTY timing
60loops, or try to grok it from external data sources. Motorola
61firmware opts for the NASTY timing loops, but needs to configure the
62serial ports to do so.
63
64
65You may have a legitimate need to define CFG_USE_OSCCLK if your
66MBX8xx board is using the OSCM clocking mode.
67
68You better know what you are doing here.