Rainbow Electronics ATAR862-3 - Manual (Page 24)

24
ATAR862-3
4556B–4BMCU–02/03
Figure 19. Internal Supply Voltage Supervisor
Figure 20. External Input Voltage Supervisor
Clock Generation
Clock Module The microcontroller block contains a clock module with 4 different internal oscillator
types: two RC-oscillators, one 4-MHz crystal oscillator and one 32-kHz crystal oscillator.
The pins OSC1 and OSC2 are the interface to connect a crystal either to the 4-MHz, or
to the 32-kHz crystal oscillator. OSC1 can be used as input for external clocks or to con-
nect an external trimming resistor for the RC-oscillator 2. All necessary circuitry, except
the crystal and the trimming resistor, is integrated on-chip. One of these oscillator types
or an external input clock can be selected to generate the system clock (SYSCL).
In applications that do not require exact timing, it is possible to use the fully integrated
RC-oscillator 1 without any external components. The RC-oscillator 1 center frequency
tolerance is better than
± 50%. The RC-oscillator 2 is a trimmable oscillator whereby the
oscillator frequency can be trimmed with an external resistor attached between OSC1
and V
DD
. In this configuration, the RC-oscillator 2 frequency can be maintained stable
with a tolerance of
±
15% over the full operating temperature and voltage range.
The clock module is programmable via software with the clock management register
(CM) and the system configuration register (SC). The required oscillator configuration
can be selected with the OS1-bit and the OS0-bit in the SC-register. A programmable
4-bit divider stage allows the adjustment of the system clock speed. A special feature of
the clock management is that an external oscillator may be used and switched on and
off via a port pin for the power-down mode. Before the external clock is switched off, the
internal RC-oscillator 1 must be selected with the CCS-bit and then the SLEEP mode
may be activated. In this state an interrupt can wake up the controller with the RC-oscil-
lator, and the external oscillator can be activated and selected by software. A
synchronization stage avoids too short clock periods if the clock source or the clock
speed is changed. If an external input clock is selected, a supervisor circuit monitors the
external input and generates a hardware reset if the external clock source fails or drops
below 500 kHz for more than 1 ms.
V
DD
Low threshold
Middle threshold
High threshold
VMS = 1
Low threshold
Middle threshold
High threshold
VMS = 0
3.0 V
2.6 V
2.2 V
1.3 V
VMI
VMS = 1
VMS = 0
Positive slope
Negative slope
VMS = 1
VMS = 0
Interrupt negative slope
Interrupt positive slope
Internal reference level
t