Circuit Operation - Yamaha FB-01 Service Manual

■ CIRCUIT OPERATION
1. CPU Clock Circuit
A 1 2MHz crystal is utilized in the clock generation circuit
to provide the clock signal for the CPU and sound source.
A 6MHz clock signal is applied to the clock terminal (pin
6) of the CPU via IC14, a divide by 2 circuit.
2. Sub-clock Circuit
A 4MHz crystal is utilized in the clock generation circuit
to provide the clock signal for the OPP, DAC, and MIDI cir¬
cuitry. It is divided down to 500KHz by IC9 (74HC161)
and applied to the USART (MPD71051) and OPP (YM2164).
3. Reset Circuit
POWER ON RESET is generated by means of the reset cir¬
cuit consisting of C87, R37, R72 and ZD1. The circuit is
also connected to the Chip Enable (CE) signal generation
circuit for the RAM battery backup function so that the
RAM data is protected at power OFF.
4. CPU, ROM, and RAM
The CPU operates on the basis that the system clock is
about 6MHz. For this reason, the access time of the bus
connecting elements such as ROM and RAM must be faster
than 1 50nS.
The ROM, from 0000H to 7FFFH, is for the voice data and
program area as well as for configuration data area.
The RAM, from 8000H to BFFFH, is used as the area for
storing various types of register and stack information.
The RAM memory is also utilized for storing the Configura¬
tion Dat^, and the User Memory Data. This RAM area is
battery backed-up in order to save or hold the saved data.
■ SYSTEM BLOCK DIAGRAM
IN THROUGH OUT
AUDIO PRE
AMPUFIER
5. The Sound Source Circuit
This circuit consists of a 4-operator, 8-voices FM sound
source (OPP YM2164) and the YM3012 DAC. The FM
sound source circuit operates according to the data from
the CPU. There are two channels for the audio output and
they can produce different voices.
6. Switch Scan Circuit
This circuit scans the ON/OFF states of the 8 switches by
means of 1C 1 8 (74HC244) which is directly connected to
the CPU data bus.
7. MIDI Circuit
The USART (MPD71051: 8251 compatible) is used as the
Transmitting and Receiving device of the MIDI circuit. It
conforms to the MIDI standard VI.0.