5 RESET AND NMI
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
5-3
Precautions to be Taken during Initial Reset
5.1.5
Core CPU
When initially reset, all internal registers of the core CPU are undefined. Therefore, these registers must be
initialized in a program. In particular, the Stack Pointer (SP) should always be initialized before accessing the
stack. Note that NMI requests are masked with hardware until data is written to the SP after an initial reset, to
prevent erratic operation.
Internal RAM
The contents of internal RAM are undefined when initially reset. Internal RAM must be initialized as required.
High-speed (OSC3) oscillator circuit
When initially reset, the high-speed (OSC3) oscillator circuit starts oscillating, and when the reset signal is
negated, the CPU starts operating with the OSC3 clock. To prevent erratic operation due to an unstable clock
when the chip is reset at power-on or while the high-speed (OSC3) oscillator circuit is idle, the reset signal
should not be negated until after oscillation stabilizes.
Low-speed (OSC1) oscillator circuit
When the chip is reset at power-on or while the low-speed (OSC1) oscillator circuit is idle, the low-speed (OSC1)
oscillator circuit also starts oscillating. The low-speed (OSC1) oscillator circuit requires a longer time for oscil-
lation to stabilize than the high-speed (OSC3) oscillator circuit. (See the electrical characteristics table.) To pre-
vent erratic operation due to an unstable clock, the OSC1 clock should not be used only after this stabilization
time elapsed.
I/O ports and I/O pins
Initial reset initializes the I/O port control and data registers, therefore, be set up back again in a program.
Other internal peripheral circuits
The control and data registers of other peripheral circuits are initialized or undefined by initial reset. Therefore,
setup of these registers with a program is required.
For the specific initial settings done on the peripheral circuits after an initial reset, see each I/O map or circuit
descriptions.
NMI Input
5.2
The S1C33L26 has two NMI sources that generate NMI.
(1) #NMI pin (external input)
(2) Watchdog timer (software selectable)
Figure 5.2.1 shows the configuration of the NMI circuit.
#NMI
NMI input signal
NMI request signal
WDT NMI signal
Watchdog
timer
2.1 Configuration of NMI Circuit
Figure 5.
The NMI signal, which is input from the #NMI pin or generated by the watchdog timer (WDT), generates a non-
maskable interrupt to the C33 PE Core. This interrupt takes precedence over other interrupts and is unconditionally
accepted by the C33 PE Core.
For details about NMI exception handling by the C33 PE Core, refer to the “S1C33 Family C33 PE Core Manual.”