at89c55-16qa ATMEL Corporation, at89c55-16qa Datasheet - Page 12

• Manufacturer Part Number: at89c55-16qa
• Description: 8-bit Microcontroller With Bytes Flash
• Manufacturer: ATMEL Corporation

the Timer 2 flag, TF2, is set at S2P2 and is polled in the
same cycle in which the timer overflows. For further infor-
mation, see the Microcontroller Data Book, section titled
“Interrupts.”
Oscillator Characteristics
XTAL1 and XTAL2 are the input and output, respectively,
of an inverting amplifier that can be configured for use as
an on-chip oscillator, as shown in Figure 7. Either a quartz
crystal or ceramic resonator may be used. To drive the
device from an external clock source, XTAL2 should be left
unconnected while XTAL1 is driven, as shown in Figure 8.
There are no requirements on the duty cycle of the external
clock signal, since the input to the internal clocking circuitry
is through a divide-by-two flip-flop, but minimum and maxi-
mum voltage high and low time specifications must be
observed.
Idle Mode
In idle mode, the CPU puts itself to sleep while all the on-
chip peripherals remain active. The mode is invoked by
software. The content of the on-chip RAM and all the spe-
cial functions registers remain unchanged during this
mode. The idle mode can be terminated by any enabled
interrupt or by a hardware reset.
Note that when idle mode is terminated by a hardware
reset, the device normally resumes program execution
from where it left off, up to two machine cycles before the
internal reset algorithm takes control. On-chip hardware
inhibits access to internal RAM in this event, but access to
the port pins is not inhibited. To eliminate the possibility of
an unexpected write to a port pin when idle mode is termi-
nated by a reset, the instruction following the one that
invokes idle mode should not write to a port pin or to exter-
nal memory.
Status of External Pins During Idle and Power Down Modes
4-180
Mode
Idle
Idle
Power Down
Power Down
Program Memory
Internal
External
Internal
External
AT89C55
ALE
1
1
0
0
Figure 7. Oscillator Connections
Note:
Figure 8. External Clock Drive Configuration
PSEN
1
1
0
0
C1,C2
PORT0
Float
Float
Data
Data
=
=
30 pF for Crystals
40 pF for Ceramic Resonators
PORT1
Data
Data
Data
Data
Address
PORT2
Data
Data
Data
PORT3
Data
Data
Data
Data