Stanford Research Systems BJT SIM911 Operation And Service Manual page 33

4.1 Circuit Discussion
SIM911 BJT Preamp
ensure that no "runt" clock pulses are produced that would violate
U405's minimum clock periods. Four separate clock-starting signals
are combined by U402:
Power-on reset
Amplifier overload
Incoming serial data
Front-panel button press
The fast start-time of the RC-oscillator ensures that incoming se-
rial data will be correctly decoded by the microcontroller's UART,
even when the clock is started by the serial start bit of the incoming
data. When the microcontroller has completed all pending activity,
it drives the STOP signal high (pin 24 of U405), e ectively halting its
own processor clock. In this way, the SIM911 guarantees no digital
clock artifacts can be generated during quiescent operation.
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