Maxim Integrated MAXQ Family Users Guide: MAXQ2000 Supplement - Manual (Page 16)

MAXQ Family User’s Guide:
MAXQ2000 Supplement
Reset Conditions
There are three possible reset sources for the MAXQ2000. While in the reset state, the enabled system clock oscillator continues run-
ning, but no code execution occurs. Once the reset condition has been removed or has completed, code execution resumes at
address 8000h for all reset types.
Power-On Reset
When power is first applied to the MAXQ2000, or when the internal supply voltage V DD drops below the minimum allowed value, the
processor is held in a power-on reset state (Figure 6). For the MAXQ2000 to exit power-on reset, the following two conditions must apply:
• V DD is within the acceptable range for that power supply (see data sheet for values).
• The ring oscillator has completed 65,536 cycles (delay for power supply to stabilize).
Note that since the MAXQ2000 has no brownout reset function, the power-on reset is only guaranteed to occur if V DD drops all the way
to ground before rising again. Brownout events where V DD drops partially (but remains above ground) and then rises back into the
acceptable range are not guaranteed to trigger a power-on reset and can result in unpredictable device behavior.
If the V DDIO power supply drops to ground, a power-on reset is not triggered. However, this causes all port pins driven by V DDIO to
drop to ground. Additionally, the 32kHz oscillator only operates when V DDIO is within acceptable limits.
Watchdog Timer Reset
The watchdog timer on the MAXQ2000 functions as described in the MAXQ Family User’s Guide. When running at 14MHz, the maxi-
mum watchdog time period before reset is approximately 150ms.
Since the RGSL bit is cleared to 0 on power-on reset only, it is possible to exit a watchdog reset with the clock source set to the high-
frequency oscillator. In this case, execution resumes running from the ring oscillator, and the switchover to the high-frequency oscilla-
tor occurs automatically when the 65,536-cycle warmup delay for that oscillator has completed.
External Reset
External reset via RESET is a synchronous reset source. After the external reset low has been removed and sampled, execution
resumes (running from the ring oscillator) following a delay of four ring-oscillator cycles, as shown in Figure 7.
Since the RGSL bit is cleared to 0 on power-on reset only, it is possible to exit an external reset with the clock source set to the high-
frequency oscillator. In this case, execution resumes running from the ring oscillator, and the switchover to the high-frequency oscilla-
tor occurs automatically when the 65,536-cycle warmup delay for that oscillator has completed.
V DD(MIN)
T1
RING
OSCILLATOR
INTERNAL
RESET
T2
(T1 = STARTUP TIME FOR RING OSCILLATOR)
(T2 = 65,536 RING OSCILLATOR CYCLES, OR 8.192ms AT 8MHz)
Figure 6. MAXQ2000 Power-On Reset
Maxim Integrated 16