17

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.

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

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:

ts

ns

=⋅

=

−

10 10

1000

7

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.