Rainbow Electronics AT89LP216 - Manual (Page 5)

5
3621A–MICRO–6/06
AT89LP216 [Preliminary]
5.3 Interrupt Handling
The interrupt controller polls the interrupt flags during the last clock cycle of any instruction. In
order for an interrupt to be serviced at the end of an instruction, its flag needs to have been
latched as active during the next to last clock cycle of the instruction, or in the last clock cycle of
the previous instruction if the current instruction executes in only a single clock cycle.
The external interrupt pins, INT0 and INT1, are sampled at every clock cycle instead of once
every 12 clock cycles. Coupled with the shorter instruction timing and faster interrupt response,
this leads to a higher maximum rate of incidence for the external interrupts.
5.4 Timer/Counters
By default the Timer/Counters is incremented at a rate of once per clock cycle. This compares to
once every 12 clocks in the standard 8051. A common prescaler is available to divide the time
base for all timers and reduce the increment rate. The TPS bits in the CLKREG SFR control the
prescaler (
Table 9-2 on page 12 ). Setting TPS = 1011B will cause the timers to count once every
12 clocks.
The external Timer/Counter pins, T0 and T1, are sampled at every clock cycle instead of once
every 12 clock cycles. This increases the maximum rate at which the Counter modules may
function.
5.5 Serial Port
The baud rate of the UART in Mode 0 is 1/2 the clock frequency, compared to 1/12 the clock fre-
quency in the standard 8051; and output data is only stable around the rising edge of the serial
clock. In should also be noted that when using Timer 1 to generate the baud rate in Mode 1 or
Mode 3, the timer counts at the clock frequency and not at 1/12 the clock frequency. To maintain
the same baud rate in the AT89LP216 while running at the same frequency as a standard 8051,
the time-out period must be 12 times longer. Mode 1 of Timer 1 supports 16-bit auto-reload to
facilitate longer time-out periods for generating low baud rates.
5.6 Watchdog Timer
The Watchdog Timer in AT89LP216 counts at a rate of once per clock cycle. This compares to
once every 12 clocks in the standard 8051. A common prescaler is available to divide the time
base for all timers and reduce the counting rate.
5.7 I/O Ports
The I/O ports of the AT89LP216 may be configured in four different modes. By default all the I/O
ports revert to input-only (tristated) mode at power-up or reset. In the standard 8051, all ports
are weakly pulled high during power-up or reset. To enable 8051-like ports, the ports must be
put into quasi-bidirectional mode by clearing the P1M0 and P3M0 SFRs. The user can also con-
figure the ports to start in quasi-bidirectional mode by disabling the Tristate-Port User Fuse.
When this fuse is disabled, P1M0 and P3M0 will reset to 00h instead of FFh and the ports will be
weakly pulled high.