6.4. Timer 2
The timer 2 in the T89C51RD2 is compatible with the 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, connected in
cascade. It is controlled by T2CON register (See Table 5) and T2MOD register (See Table 6). Timer 2 operation
is similar to Timer 0 and Timer 1. C/T2 selects FOSC/12 (timer operation) or external pin T2 (counter operation)
as the timer clock input. Setting TR2 allows TL2 to be incremented by the selected input.
Timer 2 has 3 operating modes: capture, autoreload and Baud Rate Generator. These modes are selected by the
combination of RCLK, TCLK and CP/RL2 (T2CON), as described in the ATMEL Wireless and Micrcontrollers
8-bit Microcontroller Hardware description.
Refer to the ATMEL Wireless and Micrcontrollers 8-bit Microcontroller Hardware description for the description
of Capture and Baud Rate Generator Modes.
In T89C51RD2 Timer 2 includes the following enhancements:
• Auto-reload mode with up or down counter
• Programmable clock-output
6.4.1. Auto-Reload Mode
The auto-reload mode configures timer 2 as a 16-bit timer or event counter with automatic reload. If DCEN bit
in T2MOD is cleared, timer 2 behaves as in 80C52 (refer to the ATMEL Wireless and Micrcontrollers 8-bit
Microcontroller Hardware description). If DCEN bit is set, timer 2 acts as an Up/down timer/counter as shown in
Figure 5. In this mode the T2EX pin controls the direction of count.
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 overflows or underflows according to the the direction of the count. EXF2
does not generate any interrupt. This bit can be used to provide 17-bit resolution.
Rev. F - 15 February, 2001