S3C80A5B
RESET
and POWER-DOWN
8-3
Voltage [V]
Time
Reset pulse
Va
Reset Pulse Width
V
DD
V
IH
= 0.85 V
DD
V
IL
= 0.4 V
DD
TV
DD
(V
DD
Rising Time)
V
DD
If Va voltage is under the 0.4 V
DD
, Reset pulse signal is gernerated.
If Va voltage is over than 0.4 V
DD
, Reset pulse is not gernerated.
Figure 8-3. Timing Diagram for Power-on Reset Circuit
SYSTEM RESET OPERATION
System reset starts the oscillation circuit, synchronize chip operation with CPU clock, and initialize the internal CPU
and peripheral modules. This procedure brings the S3C80A5B into a known operating status. To allow time for
internal CPU clock oscillation to stabilize, the reset pulse generator must be held to active level for a minimum time
interval after the power supply comes within tolerance. The minimum required reset operation for a oscillation
stabilization time is 16 oscillation clocks. All system and peripheral control registers are then reset to their default
hardware values (see Tables 5-1).
In summary, the following sequence of events occurs during a reset operation:
— All interrupts are disabled.
— The watchdog function (basic timer) is enabled.
— Ports 0, 1 and 2 are set to input mode and all pull-up resistors are disabled for the I/O port pin circuits.
— Peripheral control and data register settings are disabled and reset to their default hardware values (see Table 5-
1).
— The program counter (PC) is loaded with the program reset address in the ROM, 0100H.
— When the programmed oscillation stabilization time interval has elapsed, the instruction stored in ROM location
0100H (and 0101H) is fetched and executed.