Bose CD-20 Service Manual page 9

THEORY OF OPERATION
3.0 Control Electronics (schematic Diagram Sheet 3)
The audio circuits, tuner, CD mechanism, display, EEPROM, remote receiver, push buttons,
and AC power status are all overseen by microprocessor (micro) U400. The micro is clocked
by an 8 MHz ceramic resonator (X401) and is given a reset pulse by U403 (the reset IC). This
occurs whenever the +5V power supply falls below about 4.75V and automatically at power-up.
The micro communicates with all three audio chips (U100, U101, and U102) using a three wire
interface (clock, data and ground). In all three cases, the clock line is the ACLOCK line, port
P35 on the micro. Depending on which chip is being addressed, the micro may use either the
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ADATA_1 or ADATA_2 lines to send the data. The data is sent in I C format to all three chips.
The micro sends commands to the audio chips whenever new input sources are selected, or
the volume needs to change, or a zone needs to mute or unmute.
The micro changes AM and FM tuner stations by writing to the PLL (Phase-Locked Loop) chip
U302. This information is sent using the same clock and data lines used for the audio chips
(ACLOCK and ADATA_2). However, the PLL chip enable (PLL_CE) must also be high for the
chip to receive the message. The micro can tell when a station is strong enough to stop seek-
ing by monitoring the RF_SIG/ line from U300 (the LA1851 detector chip). The micro can then
check to see if that same station is transmitting a valid carrier (in case an adjacent station was
strong enough to fool it into stopping) by asserting the F_TEST line. The result of the fre-
quency test is signaled to the micro by the RF_F_OK line.
The CD-20's 6-disc CD mechanism is a complete (mechanical as well as electrical) assembly
that is purchased as an OEM unit from FMS (Ford Mazda Sanyo). It comes with its own micro-
processor-based controller PCB that is capable of many functions. The CD-20's micro commu-
nicates with this PCB via a dedicated serial data bus (CD_BUS, micro port P12). The CD-20
micro (U400) sends control commands (play, pause, stop, skip forward, etc.) to the mechanism
via the CD_BUS line, and receives disc, track, etc., information in the same way. The CD
mechanism is enabled by U400 by dropping the CD_RES (CD reset) line and raising the
CD_ENBL (CD enable) line. The appropriate control commands are then sent.
The micro controls the vacuum fluorescent display (VFD400) with its on-chip VFD driver. The
driver scans the display's grids and segments automatically using ports P7, P8 and P9. The
CD-20 display is divided into 8 groups, each enabled by a separate grid line (G1 through G8).
Each of these groups contains up to 15 icons. Each icon is enabled by a separate segment
line (S1 through S15). Although icons appear to be lit constantly, in fact each is actually turning
on and off at a rate too fast to see. Each of the 8 groups of icons are only enabled 1/8th of the
time (first group 1, then group 2, etc., in a repeating sequence). The segment lines are used to
select which icons should be lit as each icon group gets enabled. The 511 Ω series resistors
are added to reduce scan currents that might interfere with AM tuner reception. All series
resistors and inductors on the +5V, +10V, -24V and -30V power supply lines are similarly
added to reduce tuner interference.
U401 is a 1k-bit (128 bytes x 8) EEPROM (Erasable, Programmable Memory) that stores the
AM and FM station presets, RF remote House Code and background (self-test) information.
Whenever a new preset is stored or erased by the user, information gets written to U401. The
micro communicates with the EEPROM using a three wire interface that is identical (but sepa-
rate) to the interface used for the audio chips. In this case, the clock line is the EE_CLK signal,
port P03, and the data line is the EE_DATA signal, port P04. Although this data is also sent
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using the I C format, the EEPROM is busy enough to have its own interface (rather than
sharing the audio chip's interface).
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