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AT89LP428 数据表(PDF) 55 Page - ATMEL Corporation |
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AT89LP428 数据表(HTML) 55 Page - ATMEL Corporation |
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55 / 149 page  55 3654A–MICRO–8/09 AT89LP428/828 12.2 Capture Mode In the Capture mode, Timer 2 is a fixed 16-bit timer or counter that counts up from MIN to MAX. An overflow from MAX to MIN sets bit TF2 in T2CON. If EXEN2 = 1, a 1-to-0 transition at exter- nal input T2EX also causes the current value in TH2 and TL2 to be captured into RCAP2H and RCAP2L, respectively. In addition, the transition at T2EX causes bit EXF2 in T2CON to be set. The EXF2 and TF2 bits can generate an interrupt. Capture mode is illustrated in Figure 12-1. The Timer 2 overflow rate in Capture mode is given by the following equation: Figure 12-1. Timer 2 Diagram: Capture Mode 12.3 Auto-Reload Mode Timer 2 can be programmed to count up or down when configured in its 16-bit auto-reload mode. This feature is invoked by the DCEN (Down Counter Enable) bit located in the SFR T2MOD (see Table 12-4). Upon reset, the DCEN bit is set to 0 so that timer 2 will default to count up. When DCEN is set, Timer 2 can count up or down, depending on the value of the T2EX pin. The overflow and reload values depend on the Timer 2 Count Mode bits, T2CM 1-0 in T2MOD. A summary of the Auto-reload behaviors is listed in Table 12-5. Capture Mode: Time-out Period 65536 Oscillator Frequency ------------------------------------------------------- TPS 1 + () × = ÷TPS EXF2 T2EX PIN T2 PIN TR2 EXEN2 C/T2 = 0 C/T2 = 1 CAPTURE OVERFLOW TRANSITION DETECTOR TIMER 2 INTERRUPT RCAP2H RCAP2L TL2 TH2 TF2 OSC Table 12-5. Summary of Auto-reload Modes T2CM 1-0 DCEN T2EX Direction Behavior 00 0 X Up reload to BOTTOM 00 1 0 Down underflow to MAX 00 1 1 Up overflow to BOTTOM 01 0 X Up reload to MIN 01 1 0 Down underflow to TOP 01 1 1 Up overflow to MIN 10 X X Up-down and repeat 11 X X Up-down and repeat BOTTOM MAX → MAX BOTTOM → BOTTOM MAX → MIN TOP → TOP MIN → MIN TOP → MIN TOP MIN →→ MIN TOP MIN →→ |
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