CLOCK DISTRIBUTION FOR THE AMS           January 13, 1999

The AMS uses two Roboclocks. Note that skew for each output pair
is controlled by only one pair of pins, so it is has to be the same for both.
Also note the different ranges of skew adjustment with respect to feedback:
1Q0,1Q1,2Q0,2Q1 (Q00,Q10,Q01,Q11 in Concept) :  0 +-1 +-2 +-3 +-4
3Q0,3Q1,4Q0,4Q1 (Q02,Q12,Q03,Q13 in Concept) :  0 +-2 +-4 +-6
Therefore if some clock lines need to be swapped to optimise pin assignements,
care should be taken to stay in the same "skew range".
Also notice that FB is taken from the output pin which is closest to the
feeback pin, to avoid line crossing and signal reflection.

Destination loads are:	Xilinx 73144: 12pF
			Xilinx  7336:  8pf
			Cypress CY7C4245 10pF?
			Cypress 384a: 10pF
			sockets:  unknown !
			AS32 TTL:     15 pF ?

ROBOCLOCK_1 : uses the quartz oscillator as REF.
name skew   output#		destination chips		      load(pF)
        Cypress Cadence
-------------------------------------------------------------------------------
CLK1   0    1Q0	Q00      USQ_SREG1 USQ_SREG2                            16
CLK2   0    1Q1 Q10      USQ_OREG1 USQ_OREG2                            16

CLK3   0    2Q0 Q01      INPUT FIFO1 FIFO2 		                42?
CLK4   0    2Q1 Q11      OUTPUT OSPY	                                24

WCLK2  0    3Q0 Q02      AS32 for HSPYW_                                15 ?
WCLK1  0    3Q1 Q12      AS32 for OUTDS_  AS32 for OSPYW_               10 ?

CLK5   0    4Q0 Q03      VME                                            12
ICLK6  0    4Q1 Q13      ROBOCLOCK_2                                     5 ?

OUTPUT 3Q0 (Q02) (WCLK2) is used as feedback (FB pin).


ROBOCLOCK_2 : uses ICLK6 from roboclock_1 as REF.
name        skew   output#		destination chips	      load(pF)
                Cypress Cadence
------------------------------------------------------------------------------
CLOCK6      -4    1Q0 Q00      P3 connector   A2(CLOCKOUT2)             10 ?
CLOCK9	    -4    1Q1 Q10      P3 connector   A1(CLOCKOUT1=SPARECLOCK1)

SPARECLOCK2  0    2Q0 Q01
SPARECLOCK3  0    2Q1 Q11     Used as Feedback. Skew can't be changed	10  

CLOCK7       0    3Q0 Q02    INPREG GLUE                                22
CLOCK8       0    3Q1 Q12    HSPY					12

SPARECLOCK6	  4Q0 Q03     
SPARECLOCK7	  4Q1 Q13

OUTPUT 2Q1 (Q11) (SPARECLOCK3) is used as feedback (FB pin).