Smiths Medical BCI 3304, Service Manual

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Chapter 2: Oximeter Circuit Description
3304 Service Manual 2-3
Reset Circuit
Watchdog timer chip U15 performs two functions:
/RESET is generated by U15 if VCC is less than +4.4 VDC. •
/PFO (power fail output) is generated by U15 if the input PFI at •
U15-4 becomes less than 1.25 VDC. /PFO turns off the oximeter
through U18A and Q4.
Gates U19C and U19D disable access to RAM chip U13 while /RESET is
active.
Memory and I/O Decoding
Demultiplexer U16A decodes A15, A16, and ME to allocate memory
space. The following chart shows the memory address allocation:
FROM TO enABLeD DevICe
00000H 0FFFFH PROM U12.
10000H 1FFFFH RAM U13.
20000H 2FFFFH Display IC load strobe.
30000H 3FFFFH Interrupt request to microprocessor U11.
Demultiplexer U16B decodes A14, A15, and IOE to allocate I/O space.
The following chart shows the I/O address allocation:
FROM TO SIgnAL OR enABLeD DevICe
0000H 3FFFH not used.
4000H 7FFFH U20, U21 and U22 strobe signal.
8000H BFFFH /HOLD signal to A/D converted U8.
C000H FFFFH U29 strobe signal.
Dual microprocessor operation
Microprocessor U11 is a slave Digital Signal Processing (DSP)
device. It shares common PROM and RAM with the master U10
microprocessor. U17A, U17B, U17C and U17D mix control signals from
both microprocessors together. At power up U11 puts U10 on hold
using signal /BUSRQ, and boots from EPROM U12. Then it switches to
its internal data and instruction memory releasing U10. Each time U11
needs to access data from the common RAM, it sets U10 to hold. If U10
needs to inform U11, it generates /DINT interrupt signal.