Rainbow Electronics T89C5115 - Manual (Page 57)

57
T89C5115
4128A–8051–04/02
Timer 2
The T89C5115 Timer 2 is compatible with Timer 2 in the 80C52.
It is a 16-bit timer/counter: the count is maintained by two eight-bit timer registers, TH2
and TL2 that are cascade- connected. It is controlled by T2CON register (See Table )
and T2MOD register (See Table 39). Timer 2 operation is similar to Timer 0 and Timer
1. C/T2 selects F
T2 clock
/6 (timer operation) or external pin T2 (counter operation) as
timer clock. Setting TR2 allows TL2 to be incremented by the selected input.
Timer 2 includes the following enhancements:
• Auto-reload mode (up or down counter)
• Programmable clock-output
Auto-reload Mode The auto-reload mode configures Timer 2 as a 16-bit timer or event counter with auto-
matic reload. This feature is controlled by the DCEN bit in T2MOD register (See
Table 39). Setting the DCEN bit enables Timer 2 to count up or down as shown in
Figure 27. In this mode the T2EX pin controls the counting direction.
When T2EX is high, Timer 2 counts up. Timer overflow occurs at FFFFh which sets the
TF2 flag and generates an interrupt request. The overflow also causes the 16-bit value
in RCAP2H and RCAP2L registers to be loaded into the timer registers TH2 and TL2.
When T2EX is low, Timer 2 counts down. Timer underflow occurs when the count in the
timer registers TH2 and TL2 equals the value stored in RCAP2H and RCAP2L registers.
The underflow sets TF2 flag and reloads FFFFh into the timer registers.
The EXF2 bit toggles when Timer 2 overflow or underflow, depending on the direction of
the count. EXF2 does not generate an interrupt. This bit can be used to provide 17-bit
resolution.
Figure 27. Auto-reload Mode Up/Down Counter
(DOWN COUNTING RELOAD VALUE)
TF2
T2
EXF2
TH2
(8-bit)
TL2
( 8-bit)
RCAP2H
(8-bit)
RCAP2L
(8-bit)
FFh
(8-bit)
FFh
( 8-bit)
TOGGLE
(UP COUNTING RELOAD VALUE)
Timer 2
INTERRUPT
:6
T2CONreg
T2CONreg
T2EX:
1=UP
2=DOWN
0
1
CT/2
T2CON.1
TR2
T2CON.2
FT2
CLOCK
see section “Clock”