Power Management
Reset Pin
Two power reduction modes are implemented in the T89C5115: the Idle mode and the
Power-down mode. These modes are detailed in the following sections. In addition to
these power reduction modes, the clocks of the core and peripherals can be dynamically
divided by 2 using the X2 Mode detailed in Section “Clock”.
In order to start-up (cold reset) or to restart (warm reset) properly the microcontroller, a
high level has to be applied on the RST pin. A bad level leads to a wrong initialisation of
the internal registers like SFRs, PC, etc. and to unpredictable behavior of the microcon-
troller. A warm reset can be applied either directly on the RST pin or indirectly by an
internal reset source such as a watchdog, PCA, timer, etc.
At Power-up (cold reset) Two conditions are required before enabling a CPU start-up:
•
VDD must reach the specified VDD range,
•
The level on xtal1 input must be outside the specification (VIH, VIL).
If one of these two conditions are not met, the microcontroller does not start correctly
and can execute an instruction fetch from anywhere in the program space. An active
level applied on the RST pin must be maintained until both of the above conditions are
met. A reset is active when the level VIH1 is reached and when the pulse width covers
the period of time where VDD and the oscillator are not stabilized. Two parameters have
to be taken into account to determine the reset pulse width:
•
•
VDD rise time (vddrst),
Oscillator startup time (oscrst).
To determine the capacitor the highest value of these two parameters has to be chosen.
The reset circuitry is shown in Figure 5.
Figure 5. Reset Circuitry
VDD
Crst
RST pin
Internal reset
Rrst
Reset input circuitry
0
Table 12 and Table 13 give some typical examples for three values of VDD rise times,
two values of oscillator start-up time and two pull-down resistor values.
Table 12. Minimum Reset Capacitor for a 50K Pull-down Resistor
oscrst/vddrst
5ms
1ms
10ms
1.2µF
3.9µF
100ms
12µF
820nF
2.7µF
20ms
12µF
18
AT89C5115
4128F–8051–05/06
ATMEL [ ATMEL ]
