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its latter role, IC1 contains a PLL circuit to lock to the incoming
digital audio data rate. A secondary support chip, IC3, initiates a
power-on reset to both IC1 and DSP IC2.
Both IC1 and IC2 operate from a +3.3V supply rail, which is deliv-
ered by se3.3A (analog) and +3.3D (digital) power buses,
which eventually go back to the +3.3V switching regulator. In addi-
tion, IC2 requires yet another power supply source, but thankfully
supervises its own switching supply, requiring only a MOSFET, Q1,
and coil L1.
PROGRAM AND SETTINGS MEMORY
IC12 on the main board is a serial EEPROM memory chip. At
power-up, the contents of this memory are loaded into the DSP.
This includes the processing algorithms and all user settings, which
are loaded back into this chip as they are made.
IC12 provides an easy and relatively inexpensive means of issuing
updates for the 261. Replacing this plug-in IC amounts to firm-
ware updating, as opposed to software updating, which would be
accomplished with a download or from a disk. Omitting a communi-
cations port in the Model 261 has greatly simplified the design, and
was judged not to pose a compromise in an otherwise uncomplicated
product.
