Rainbow Electronics AT89C51SND1C - Manual (Page 53)

53
AT8xC51SND1C
4109E–8051–06/03
Mode 1 (16-bit Timer) Mode 1 configures Timer 0 as a 16-bit Timer with TH0 and TL0 registers connected in
cascade (see Figure 31). The selected input increments TL0 register. Figure 32 gives
the overflow period calculation formula when in timer mode.
Figure 31. Timer/Counter x (x = 0 or 1) in Mode 1
Figure 32. Mode 1 Overflow Period Formula
Mode 2 (8-bit Timer with Auto-
Reload) Mode 2 configures Timer 0 as an 8-bit Timer (TL0 register) that automatically reloads
from TH0 register (see Table 62). TL0 overflow sets TF0 flag in TCON register and
reloads TL0 with the contents of TH0, which is preset by software. When the interrupt
request is serviced, hardware clears TF0. The reload leaves TH0 unchanged. The next
reload value may be changed at any time by writing it to TH0 register. Figure 34 gives
the autoreload period calculation formula when in timer mode.
Figure 33. Timer/Counter x (x = 0 or 1) in Mode 2
Figure 34. Mode 2 Autoreload Period Formula
Mode 3 (2 8-bit Timers) Mode 3 configures Timer 0 such that registers TL0 and TH0 operate as separate 8-bit
Timers (see Figure 35). This mode is provided for applications requiring an additional 8-
bit Timer or Counter. TL0 uses the Timer 0 control bits C/T0# and GATE0 in TMOD reg-
ister, and TR0 and TF0 in TCON register in the normal manner. TH0 is locked into a
Timer function (counting F
TF1
/6) and takes over use of the Timer 1 interrupt (TF1) and
run control (TR1) bits. Thus, operation of Timer 1 is restricted when Timer 0 is in mode
TRx
TCON Reg
TFx
TCON Reg
0
1
GATEx
TMOD Reg
Overflow
Timer x
Interrupt
Request
C/Tx#
TMOD Reg
TLx
(8 bits)
THx
(8 bits)
INTx
Tx
TIMx
CLOCK
÷ 6
6
⋅
(65536 – (THx, TLx))
TFx
PER
=
F
TIMx
TRx
TCON reg
TFx
TCON reg
0
1
GATEx
TMOD reg
Overflow
Timer x
Interrupt
Request
C/Tx#
TMOD reg
TLx
(8 bits)
THx
(8 bits)
INTx
Tx
TIMx
CLOCK
÷ 6
TFx
PER
=
F
TIMx
6
⋅
(256 – THx)