TLC770X Datasheet, PDF (17/27 Pages) Texas Instruments – BiCMOS SUPPLY VOLTAGE SUPERVISORS

4. The Behavior of Digital Circuits
The behavior of the circuit which is being monitored, and the switch on times which result, must be understood in
order to make correct use of the Supply Voltage Supervisor TLC770X.
The delay time t_d which must be adjusted on these components depends on the time which the system requires to
become stable after the supply voltage has been switched on.
The circuit which is being monitored must be switched into an active state only when all parts of the circuit are fully
operational.
At this point special attention must be paid to two factors.
The oscillators used in the circuit (usually crystal oscillators) must be oscillating properly before the Reset signal
becomes inactive. The higher the quality factor of such an oscillator (i.e. the lower the damping), the longer will be
the time to begin oscillating. In the case of very good oscillator circuits, this can be in the order of several 10’s of ms.
For this period of time, no precise reaction of the system being monitored can be predicted.
The Reset circuit of the microprocessor is the second factor which must be taken into account when choosing the
delay time t_d which will be used.
If a Low level is applied to the RESET − Input of a microprocessor, then a Reset program will be initiated which
brings the processor into a defined initial state. This Reset signal must therefore be applied to the Reset input for a
time long enough for the program to have been fully completed, and for the system to have reached a defined state.
With a TMS320C30 (see Figure 15.) the processing of the Reset program lasts for 10 clock cycles of the internal
clock pulse H1. The clock cycle of H1 is double as long as the clock cycle of the oscillator CLKIN. With a clock
frequency of the external clock CLKIN of 20 MHz, the time t which results is:
t =10⋅10^−7 s =1000 ns
Figure 15. RESET procedure with the TMS370
The RESET signal must therefore be applied for at least 1000 ns, in order to bring the system into a defined initial
state.
In accordance with the considerations discussed above, the RESET signal must be applied at least until the supply
voltage has reached its nominal value, until the oscillator is correctly oscillating, and until the initialization program of
the processor has been completed. Only after this time has elapsed can correct operation of the system be assured.
In practice, the delay time will therefore be chosen to be several 100’s of ms.
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