Philips SCOPEMETER PM93 Service Manual page 44

3-8
CIRCUIT DESCRIPTIONS
Analog ASIC bus
The Analog ASIC (A-ASIC 02301, see circuit diagram A2aJA2b, figure 10.5/10.6) or A-ASIC, as used
in the following text, is controlled by the microprocessor. The microprocessor uses the signals COAT,
CCLK and OTAEa,b,c to set the A-ASIC and the attenuator sections on the analog A2 PCB. These
signals together form the CONTROL bus.
Flash ROM type selection
The ScopeMeter hardware allows the usage of different types of Flash ROMs. The actual Flash ROM
configuration is indicated by resistors R1222 and R1224.
FLASH ROM CONFIGURATION
Resistor(s) F512 + F256 F010(1x)
R1222
* *
R1224
*
The resulting voltage levels (0 volt, 2.5 volt or 5 volt) are read directly by the microprocessor AlO
converter inputs.
ON/OFF circuit
The ON/OFF circuit operates almost like a thyristor. When the ON/OFF key is pressed, a current is
drawn from the base of V1503, via R1503 and V1501. Transistor V1503 will now start to conduct. This
results in a current through R1507, R1504, V1502 and R1506. The signal POWER_ON will now
become "high". Also transistor V1506 will conduct, supplying base current to V1503 after the ON/OFF
key is released. The POWER-ON signal will latch "high". The ON/OFF signal will go high, turning off
V1506 and V1503, the next time the ON/OFF key is depressed. The POWER_ON signal will become
"low" and the ScopeMeter power turns off.
RESET circuit
The RESET circuit consists of V1203,
V1205, V1215, V1201, 01205 and related
components. When the Scope Meter
power is switched on, the +5V supply
voltage starts to rise. This causes the
zener diode V1202 to conduct. After some
time transistor V1203 also starts to
conduct.
R1204 and C1203 form a time delay (see
figure 3.3).
The RESET signal now is buffered by
01205 and connected with the RESET
_~----
+5 Volt supply voltage
:
4.5VOlt~
-t----
~Vt
:
voltage across C1203
_
_ _
--~:
_ I _ _ _ _ _
- .-- RESET-
: I
switch
on moment
: I
inputs of the microprocessor and the Figure 3.5 RESET signal timing
O-ASIC circuitry.
After a reset, the voltage on the EA (External Address) input of the microprocessor (pin 14) is "high".
The Microprocessor starts up using the internal Mask ROM software. First the Flash ROMs are
checked to see if they contain valid software. If this is true, output pin 6 of flip-flop 01202 is set "low".
Now the microprocessor invokes a software reset. Because of the "low" voltage on the EA input of
the microprocessor, the microprocessor will "start up" again, using the external Flash ROM software.
The reset pulse is blocked by transistor V1201 to prevent the RESET signal from performing a "hard-
reset" on the microprocessor again. At this software reset, the microprocessor enables the LCO by
means of the signal LCOPWR. ThEm the buffers that control the LCO contain valid data.