Bose BUILT-INvisible TA-1 Service Manual page 18

Detailed Theory of Operation (see schematic diagram SD256431)
Digital Signal Processor (DSP) Board, Sheet 2 of 4 (continued)
Q200 [C2] allows the microcontroller to pull down on the DSP's reset line. When the
microcontroller first boots, all of its output ports are set to a high impedance state. Q200 remains
turned on by R252, which holds the DSPs in reset, until the microcontroller is ready to boot the
DSPs.
R246, R248, [C/D2] etc. connect several of the microcontroller output ports to the MUTE inputs
on the power amplifiers. The mute signals are organized into three groups; (bass, left, right),
(center) and (left_surround, right_surround), corresponding to the three speaker modes the
product supports. Each line is EMI filtered. Pulldown resistors R813, R814 and R815 [amplifier
schematic sheet 2, [D5] guarantee that the signals will assume a LOW state i.e., muted) at
power-up.
Sheet 3 of 4 (DSPs)
There are only two DSPs, but since they each have 240 pins, they've been broken into several
different schematic symbols so as not to clutter the page:
- address and data busses.
- buss control, and miscellaneous.
- link ports (sheet 4).
- power and ground (sheet 4).
Without going into any detail, the DSPs are connected to each other and to the boot PROM
U401 [C2], as follows:
- address buss connected to address buss.
- data buss connected to data buss.
- buss control signals connected to buss control signals.
- power pins connected to 3.3V power plane.
- ground pins connected to ground plane.
The large number of interconnects and the fact that each signal has extremely fast rise times
(2 nsec), required the use of a four layer PCB. One of the inner layers is a (more or less) solid
ground plane; the other is used for a power plane, plus additional signal interconnect where
required.
U400 [B2] and associated components form a third-overtone 40 MHz oscillator. There are two
buffered outputs; one drives both DSPs and one is connected to the test connector J401 [B1],
which was used to help debug the prototype PCBs but is no longer loaded.
Sheet 4 of 4
U500 [C/D4] and associated components form a 3.3V regulated supply. C500 and L500 [C5] are
the regulator's output filter. R500 and C501 [D4] set the regulator's switching frequency (about
100 kHz). C503 [C4] is the "bootstrap" capacitor required for establishing an internal bias voltage
higher than the supply voltage (for turning on the internal N-channel MOSFET switch). C515,
L501, and C505 [C4] prevent switching harmonics from leaking back out onto the raw DC input
voltage. R501, C507, C508 [C3], etc., are feedback and compensation components.
THEORY OF OPERATION
18