Commodore 1351 Guia Del Usuario página 37

SID REGISTER CAUTIONS:
in the C64 & C128, the SID pot lines are connected to both joy-
stick ports. A 4066 analog switch is used to switch the POT
lines between the two ports based on one of the keyboard scan
lines. The means that the normal keyscan interrupt temporarily
affects the values returned in the POT registers. Therefore, in
order to perform reliable conversions, the POT lines must be
connected to the mouse for a period > 1.6 ms before the value
returned in the POT register is valid.
The best way to insure this is to wedge the mouse driver soft-
ware into the IRQ handler prior to the polled keyscan. This
more-or-less assures that the keyscan lines have been suffi-
ciently stable before the POT register is read by the mouse
drivers.
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