3. Figure 32 gives the autoreload period calculation formulas for both TF0 and TF1
flags.
Figure 33. Timer/Counter 0 in Mode 3: 2 8-bit Counters
TIM0
CLOCK
Timer 0
Interrupt
Request
÷ 6
0
1
Overflow
TL0
(8 bits)
TF0
TCON.5
T0
C/T0#
TMOD.2
INT0
GATE0
TMOD.3
TR0
TCON.4
Timer 1
Interrupt
Request
Overflow
TIM0
CLOCK
TH0
(8 bits)
÷ 6
TF1
TCON.7
TR1
TCON.6
Figure 34. Mode 3 Overflow Period Formula
6 ⋅ (256 – TH0)
6 ⋅ (256 – TL0)
TF1PER
=
TF0PER
=
FTIM0
FTIM0
Timer 1
Timer 1 is identical to Timer 0 except for Mode 3 which is a hold-count mode. The fol-
lowing comments help to understand the differences:
•
Timer 1 functions as either a Timer or event Counter in three modes of operation.
Figure 27 through Figure 31 show the logical configuration for modes 0, 1, and 2.
Timer 1’s mode 3 is a hold-count mode.
•
Timer 1 is controlled by the four high-order bits of TMOD register (see Figure 63)
and bits 2, 3, 6 and 7 of TCON register (see Figure 62). TMOD register selects the
method of Timer gating (GATE1), Timer or Counter operation (C/T1#) and mode of
operation (M11 and M01). TCON register provides Timer 1 control functions:
overflow flag (TF1), run control bit (TR1), interrupt flag (IE1) and interrupt type
control bit (IT1).
•
•
Timer 1 can serve as the Baud Rate Generator for the Serial Port. Mode 2 is best
suited for this purpose.
For normal Timer operation (GATE1 = 0), setting TR1 allows TL1 to be incremented
by the selected input. Setting GATE1 and TR1 allows external pin INT1 to control
Timer operation.
•
•
Timer 1 overflow (count rolls over from all 1s to all 0s) sets the TF1 flag generating
an interrupt request.
When Timer 0 is in mode 3, it uses Timer 1’s overflow flag (TF1) and run control bit
(TR1). For this situation, use Timer 1 only for applications that do not require an
interrupt (such as a Baud Rate Generator for the Serial Port) and switch Timer 1 in
and out of mode 3 to turn it off and on.
•
It is important to stop the Timer/Counter before changing modes.
54
AT8xC51SND2C
4341D–MP3–04/05
ATMEL [ ATMEL ]
