AT89C51ED2-RDTUM datasheet(30/137 Pages) ATMEL

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4235K–8051–05/08

AT89C51RD2/ED2

12. Timer 2

The Timer 2 in the AT89C51RD2/ED2 is the standard C52 Timer 2. It is a 16-bit timer/counter:

the count is maintained by two eight-bit timer registers, TH2 and TL2 are cascaded. It is con-

trolled by T2CON (Table 12-1) and T2MOD (Table 12-2) registers. Timer 2 operation is similar to

Timer 0 and Timer 1. C/T2 selects FOSC/12 (timer operation) or external pin T2 (counter opera-

tion) as the timer clock input. Setting TR2 allows TL2 to increment 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).

Refer to the Atmel 8-bit Microcontroller Hardware Manual for the description of Capture and

Baud Rate Generator Modes.

Timer 2 includes the following enhancements:

• Auto-reload mode with up or down counter

• Programmable clock-output

12.1

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 C51

Microcontroller Hardware Manual). If DCEN bit is set, Timer 2 acts as an Up/down timer/counter

as shown in Figure 12-1. 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 under-

flow sets TF2 flag and reloads FFFFh into the timer registers.

The EXF2 bit toggles when Timer 2 overflows or underflows according to the direction of the

count. EXF2 does not generate any interrupt. This bit can be used to provide 17-bit resolution.

AT89C51ED2-RDTUM 数据表(PDF) 30 Page - ATMEL Corporation