HP 3457A Service Manual page 187

© Sets address lines Al4 high and A1I5 low to enable decoder U513.
The low portion of timing signal E+d (U597b, pin 4) activates the NVRAM output of decoder U513. The
NVRAM signal, applied to the emitter of Q501, causes a negative pulse at the collector (NVCS) to enable
RAM USII. The output of gate U595d is pulsed low by the positive portion of the @ timing signal
(U501, pin 35) and the inverted R/W signal from inverter U599e. This causes the output of US91a (nvwr)
to go high, During this time, the low NVE signal ensures a low output from gate U596a. The nvwr signal
is applied to the base of Q500 through gate U594b, causing it to conduct. The low output of Q500
(NVWE) activates the input of RAM US511. The microprocessor then writes the information to USII
through data bus "A", lines DO through D7.
After completing the write to CAL-RAM, the microprocessor toggles the TNOP signal at US10, pin 19.
The positive TNOP pulse is applied to gate U59Ic. The negative pulse from the output of US9I¢ presets
J-K flip-flop U592b, setting its 0 output low. The low @ output from U592b clears J-K flip-flop U592:
setting its 9 output high to remove the RAM write enable signal (NVE) and "lock" the circuit. The TNoP
signal also resets counters U514 and U515.
The CAL-RAM Lock operation and CAL-RAM Write operation is performed a total of five times for
each calibration command. The Calibration Reset operation; however, is performed only once.
8-48. Reset Circuit Operation
The purpose of the reset circuit is to set the microprocessor to a known state. The reset circuit is active
under the following conditions:
© At "power-on",
© When the +5 volt power supply is low.
© If the microprocessor goes to a non-operational state.
© At the beginning of a calibration or auto calibration routine.
Power-on Reset. The power-on reset circuit consists of comparators U519b and U519d, buffer U594d
and battery BTSOL.
The minus input of comparator U519b is referenced to +3 volts by battery BT501. The plus input of
US519b is connected through a voltage divider to the +5 volt un-regulated power supply. At initial
power-on, the plus input to U519b is less than the +3 volt reference on the minus input and the output
is held low. The low output of U5I9b is applied to the plus input of U519d, holding its output low.
The low output of U519d is coupled through buffer U594d to the RESET input of the microprocessor.
As the power supply reaches operating voltage, the voltage at the plus input of U519b exceeds the
reference voltage at the minus input and the output goes positive. The positive output of USI9b ap-
plied to the plus input of comparator U519d causes its output to go high. The high output of US19d is
coupled through buffer U594d to the RESET input of the microprocessor to release it from the R At
state.
Low Power Reset. If the un-regulated +5 volt power supply should drop below approximately +7
volts, the plus input of comparator U519b drops below the +3 volt reference at the minus input causing
the output to go low. The low output of U519b is applied to the plus input of U519d causing its out-
put to go low. The low output of U519d is coupled through buffer U594d to the RESET input of the
microprocessor. The processor is held in a RESET condition until the un-regulated +5 volt supply
returns to normal.
HP 3457A Multimeter
8-22