at89c55-12qi ATMEL Corporation, at89c55-12qi Datasheet - Page 11

• Manufacturer Part Number: at89c55-12qi
• Description: 8-bit Microcontroller With Bytes Flash
• Manufacturer: ATMEL Corporation

Programmable Clock Out
A 50% duty cycle clock can be programmed to come out
on P1.0, as shown in Figure 5. This pin, besides being a
regular I/0 pin, has two alternate functions. It can be pro-
grammed to input the external clock for Timer/Counter 2 or
to output a 50% duty cycle clock ranging from 61 Hz to 4
MHz at a 16 MHz operating frequency.
To configure the Timer/Counter 2 as a clock generator, bit
C/T2 ( T 2 C O N . 1 ) m u s t b e c l e a r e d a n d b i t T2 O E
(T2MOD.1) must be set. Bit TR2 (T2CON.2) starts and
stops the timer.
The clock-out frequency depends on the oscillator fre-
quency and the reload value of Timer 2 capture registers
(RCAP2H, TCAP2L), as shown in the following equation:
Clock Out Frequency
In the clock-out mode, Timer 2 roll-overs will not generate
an interrupt. This behavior is similar to when Timer 2 is
used as a baud-rate generator. It is possible to use Timer
2 as a baud-rate generator and a clock generator simulta-
neously. Note, however, that the baud-rate and clock-out
frequencies cannot be determined independently from
one another since they both use RCAP2H and RCAP2L.
UART
The UART in the AT89C55 operates the same way as the
UART in the AT89C51 and AT89C52. For further informa-
tion, see the Microcontroller Data Book, section titled, "Se-
rial Interface."
Interrupts
The AT89C55 has a total of six interrupt vectors: two ex-
ternal interrupts (INT0 and INT1), three timer interrupts
(Timers 0, 1, and 2), and the serial port interrupt. These
interrupts are all shown in Figure 6.
Each of these interrupt sources can be individually en-
abled or disabled by setting or clearing a bit in Special
Function Register IE. IE also contains a global disable bit,
EA, which disables all interrupts at once.
Note that Table 5 shows that bit position IE.6 is unimple-
mented. In the AT89C51 and AT89C52, bit position IE.5 is
also unimplemented. User software should not write 1s to
these bit positions, since they may be used in future AT89
products.
Timer 2 interrupt is generated by the logical OR of bits TF2
and EXF2 in register T2CON. Neither of these flags is
cleared by hardware when the service routine is vectored
to. In fact, the service routine may have to determine
whether it was TF2 or EXF2 that generated the interrupt,
and that bit will have to be cleared in software.
4 x [65536
Oscillator Frequency
RCAP2H, RCAP2L ]
(continued)
Table 5. Interrupt Enable (IE) Register
Figure 6. Interrupt Sources
Symbol
EA
—
ET2
ES
ET1
EX1
ET0
EX0
User software should never write 1s to
unimplemented bits, because they may be used in
future AT89 products.
(MSB)
Enable Bit = 1 enables the interrupt.
Enable Bit = 0 disables the interrupt.
EA
Position
—
IE.7
IE.6
IE.5
IE.4
IE.3
IE.2
IE.1
IE.0
ET2 ES ET1 EX1 ET0 EX0
Disables all interrupts. If EA = 0,
no interrupt is acknowledged. If
EA = 1, each interrupt source is
individually enabled or disabled
by setting or clearing its enable
bit.
Reserved.
Timer 2 interrupt enable bit.
Serial Port interrupt enable bit.
Timer 1 interrupt enable bit.
External interrupt 1 enable bit.
Timer 0 interrupt enable bit.
External interrupt 0 enable bit.
AT89C55
Function
(LSB)
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