mirror of
https://github.com/AsahiLinux/u-boot
synced 2024-12-26 21:13:48 +00:00
69 lines
2.9 KiB
Text
69 lines
2.9 KiB
Text
CPU module revisions
|
|
--------------------
|
|
|
|
My cpu module has the model number "CMA286-60-990526-01". My motherboard
|
|
has the model number "CMA102-32M-990526-01". These are both fairly old,
|
|
and may not reflect current design. In particular, I can see from the
|
|
Cogent web site that the CMA286 has been significantly redesigned - it
|
|
now has on board RAM (4M), ethernet 10baseT PHY (on SCC2), 2 serial ports
|
|
(SMC1 and SMC2), and 48pin DIN for the FEC (if present i.e. MPC860T), and
|
|
also the EPROM is 512K.
|
|
|
|
My CMA286-60 has none of this, and only 128K EPROM. In addition, the CPU
|
|
clock is listed as 66MHz, whereas mine is 33.333MHz.
|
|
|
|
Clocks
|
|
------
|
|
|
|
Quote from my "CMA286 MPC860/821 User's Manual":
|
|
|
|
"When setting up the Periodic Interrupt Timer (PIT), be aware that the
|
|
CMA286 places the MPC860/821 in PLL X1 Mode. This means that we feed
|
|
a 25MHz clock directly into the MPC860/821. This mode sets the divisor
|
|
for the PIT to be 512. In addition, the Time Base Register (TMB)
|
|
divisor is set to 16."
|
|
|
|
I interpreted this information to mean that EXTCLK is 25MHz and that at
|
|
power on reset, MODCK1=1 and MODCK2=0, which selects EXTCLK as the
|
|
source for OSCCLK and PITRTCLK, sets RTDIV to 512 and sets MF (the
|
|
multiplication factor) to 1 (I assume this is what they mean by X1
|
|
mode above). MF=1 means the cpus internal clock runs at the same
|
|
rate as EXTCLK i.e. 25MHz.
|
|
|
|
Furthermore, since SCCR[TBS] (the Time Base Source selector bit in the
|
|
System Clock and Reset Control register) is set in the cpu initialisation
|
|
code, the TMBCLK source is forced to be GCLK2 and the TMBCLK prescale is
|
|
forced to be 16. This results in TMBCLK=1562500.
|
|
|
|
One problem - since PITRTCLK source is EXTCLK (25Mhz) and RTDIV is 512,
|
|
PITRTCLK will be 48828.125 (huh?). Another quote from the MPC860 Users
|
|
Manual:
|
|
|
|
"When used by the real-time clock (RTC), the PITRTCLK source is first
|
|
divided as determined by RTDIV, and then divided in the RTC circuits by
|
|
either 8192 or 9600. Therefore, in order for the RTC to count in
|
|
seconds, the clock source must satisfy:
|
|
|
|
(EXTCLK or OSCM) / [(4 or 512) x (8192 or 9600)] = 1
|
|
|
|
The RTC will operate with other frequencies, but it will not count in
|
|
units of seconds."
|
|
|
|
Therefore, the internal RTC of the MPC860 is not going to count in
|
|
seconds, so we must use the motherboard RTC (if we need a RTC).
|
|
|
|
I presume this means that they do not provide a fixed oscillator for
|
|
OSCM. The code in get_gclk_freq() assumes PITRTCLK source is OSCM,
|
|
RTDIV is 4, and that OSCM/4 is 8192 (i.e. a ~32KHz oscillator). Since
|
|
the CMA286-60 doesn't have this (at least mine doesn't) we can't use
|
|
the code in get_gclk_freq().
|
|
|
|
Finally, it appears that the internal clock in my CMA286-60 is actually
|
|
33.333MHz. Which makes TMBCLK=2083312.5 (another huh?) and
|
|
PITRTCLK=65103.515625 (bloody hell!).
|
|
|
|
If anyone finds anything wrong with the stuff above, I would appreciate
|
|
an email about it.
|
|
|
|
Murray Jensen <Murray.Jensen@csiro.au>
|
|
21-Aug-00
|