When the port latch contains a 0, all pFETS in figure 4 are
off while the nFET is turned on. When the port latch
makes a 0-to-1 transition, the nFET turns off. The strong
pFET, T1, turns on for two oscillator periods, pulling the
output high very rapidly. As the output line is drawn high,
pFET T3 turns on through the inverter to supply the IOH
source current. This inverter and T form a latch which
holds the 1 and is supported by T2.
When Port 2 is used as an address port, for access to
external program of data memory, any address bit that
contains a 1 will have his strong pullup turned on for the
entire duration of the external memory access.
When an I/O pin on Ports 1, 2, or 3 is used as an input, the
user should be aware that the external circuit must sink
current during the logical 1-to-0 transition. The
maximum sink current is specified as ITL under the D.C.
Specifications. When the input goes below
approximately 2 V, T3 turns off to save ICC current. Note,
when returning to a logical 1, T2 is the only internal
pullup that is on. This will result in a slow rise time if the
user’s circuit does not force the input line high.
Figure 5. Crystal Oscillator.
To drive the device from an external clock source,
XTAL1 should be driven while XTAL2 is left
unconnected as shown in figure 6. 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
maximum high and low times specified on the Data Sheet
must be observed.
Figure 6. External Drive Configuration.
XTAL1 and XTAL2 are the input and output respectively,
of an inverting amplifier which is configured for use as an
on-chip oscillator, as shown in figure 5. Either a quartz
crystal or ceramic resonator may be used.
Same as for the 80C51, plus a third timer/counter :
Timer/Event Counter 2
Timer 2 is a 16 bit timer/counter like Timers 0 and 1, it
can operate either as a timer or as an event counter. This
is selected by bit C/T2 in the Special Function Register
T2CON (Figure 1). It has three operating modes :
“capture”, “autoload” and “baud rate generator”, which
are selected by bits in T2CON as shown in Table 2.
In the capture mode there are two options which are
selected by bit EXEN2 in T2CON; If EXEN2 = 0, then
Timer 2 is a 16 bit timer or counter which upon
overflowing sets bit TF2, the Timer 2 overflow bit, which
can be used to generate an interrupt. If EXEN2 = 1, then
Timer 2 still does the above, but with the added feature
that a 1-to-0 transition at external input T2EX causes the
current value in the Timer 2 registers, TL2 and TH2, to
be captured into registers RCAP2L and RCAP2H,
respectively, (RCAP2L and RCAP2H are new Special
Function Register in the 80C52). In addition, the
transition at T2EX causes bit EXF2 in T2CON to be set,
and EXF2, like TF2, can generate an interrupt.
Table 2 : Timer 2 Operating Modes.
16 bit auto-reload
16 bit capture
baud rate generator
Rev. E (31/08/95)