HP 3457A Service Manual page 191

U635h clears flip-flop U635a to "lock" the circuit, Also, if the 4096 usec. signal is
low when the CALREQ signal is applied to the clock input of U635a, the T output will
remain high (locked ).
CAL-RAM Write operation. Once the CAL-RAM Lock ci
microprocessor:
uit has been "unlocked" (NVE set low), the
© Sets the address code of the memory location where the information is to be stored onto
address lines AO through Al2. (When writing to the protected section of U603, address lines
A9-A12 will be set low.)
Since NVE is forced low by the CAL-RAM Lock circuitry the output of AND gate U622b is forced low.
The low output of gate U622b connects to NOR gate U624c causing the output of the gate be equal to
the gate's other input, timing signal €+0. When the timing signal is in a low state, the input to the
memory location specified by address AO-A12 is activated. The microprocessor then writes the infor-
mation to U603 through data lines DO through D7.
After completing the write to CAL-RAM, the microprocessor toggles the OTC signal at U6I1, pin 19.
The positive OTt pulse is applied to gate U625d. The negative pulse from the output of U625d presets
J-K flip-flop U635b, setting its @ output low. The low @ output from U635b resets J-K flip-flop U635a,
setting its 0 output high to remove the RAM write enable signal (NVE) and "lock" the circuit. The ort
signal also resets counters U631, U632a and U632b.
The CAL-RAM Lock and CAL-RAM Write operations are performed a total of five times for each
calibration command. However, the Calibration Reset operation is performed only once.
In summary there are two ways to enable the write function of U603, one used for writing to the un-
protected section of RAM and the other for the protected section. When writing to the unprotected
section of RAM, only the address of the memory location to be written to needs to be placed on the
address bus. When writing to the unprotected section, the address needs to be on the address bus and
the CAL-RAM lock circuit must be "unlocked".
8-54, 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 U636a and U636c, inverters
U634a and U634b and battery BTOOL.
The minus input of comparator U636a is referenced to +3 volts by battery BT601. The plus input of
U636a is connected through a voltage divider to the +5 volt un-regulated power supply. At initial
power-on, the plus input to U636a is less than the +3 volt reference on the minus input and the output
is held low. The low output of U636a is applied to the plus input of U636c, holding its output low.
The low output of U636c is coupled through inverters U634a and U634b to the RESET input of the
microprocessor.
HP 3457A Multimeter
8-26