board/cogent/README.cma286

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@cmst.csiro.au>
21-Aug-00