Atari 1040STE, Field Service Manual

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2.3 INPUT/OUTPUT SUBSYSTEMS
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Intelligent keyboard with 2 button mouse/joystick interface
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Parallel printer interface
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RS232C serial interface
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Floppy disk drive and connector for external drive
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Game controller ports
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Hard disk drive interface (ACSI)
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Musical instrument network communication: Musical Instrument Digital
Interface (MIDI)
2.4 MICROPROCESSING UNIT
The STE uses the Motorola MC68000 16–bit external/32–bit internal data bus. 24–bit
address bus microprocessor, running at 8 MHz.
2.5 GSTMCU
GSTMCU is such an important component that it is involved in nearly every operation in the
computer. The functions may be summarized as follows:
CLOCK DIVIDERS takes the 16 MHz clock and outputs 8 MHz, 4 MHz, and
500 kHz clocks.
VIDEO TIMING Blank, DE (Display Enable), Vsync and Hsync are used to generate
signals for the video display. There is a Read/Write register in GSTMCU which may
be written to configure for 50 or 60 Hz operation or 71 Hz monochrome operation
(done by the Operating System).
INTERRUPT PRIORITY interrupts from the MFP and video timing are coded into
four levels of priority on outputs IPLO and IPL1 to the 68000. These levels corres-
pond to no interrupts, MFP interrupts, VSYNC interrupt, HSYNC interrupt.
level 7 reserved
level 6 MFP
level 5 unused
level 4 VSYNC
level 3 unused
level 2 HSYNC
level 1 unused
SIGNAL AND BUS ARBITRATION
GSTMCU decodes addresses to generate chip
selects to the 6850s, MFP, DMA Controller. Programmable Sound Generator,
Memory Controller, and ROMS. It receives signals from the MFP, DMA, Memory