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AT89S53-24JI Datasheet(PDF) 10 Page - ATMEL Corporation |
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AT89S53-24JI Datasheet(HTML) 10 Page - ATMEL Corporation |
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10 / 32 page  AT89S53 4-226 Timer 0 and 1 Timer 0 and Timer 1 in the AT89S53 operate the same way as Timer 0 and Timer 1 in the AT89C51, AT89C52 and AT89C55. For further information, see the October 1995 Microcontroller Data Book, page 2-45, section titled, “Timer/Counters.” Timer 2 Timer 2 is a 16 bit Timer/Counter that can operate as either a timer or an event counter. The type of operation is selected by bit C/T2 in the SFR T2CON (shown in Table 2). Timer 2 has three operating modes: capture, auto-reload (up or down counting), and baud rate generator. The modes are selected by bits in T2CON, as shown in Table 8. Timer 2 consists of two 8-bit registers, TH2 and TL2. In the Timer function, the TL2 register is incremented every machine cycle. Since a machine cycle consists of 12 oscil- lator periods, the count rate is 1/12 of the oscillator fre- quency. In the Counter function, the register is incremented in response to a 1-to-0 transition at its corresponding external input pin, T2. In this function, the external input is sampled during S5P2 of every machine cycle. When the samples show a high in one cycle and a low in the next cycle, the count is incremented. The new count value appears in the register during S3P1 of the cycle following the one in which the transition was detected. Since two machine cycles (24 oscillator periods) are required to recognize a 1-to-0 transi- tion, the maximum count rate is 1/24 of the oscillator fre- quency. To ensure that a given level is sampled at least once before it changes, the level should be held for at least one full machine cycle. Capture Mode In the capture mode, two options are selected by bit EXEN2 in T2CON. If EXEN2 = 0, Timer 2 is a 16 bit timer or counter which upon overflow sets bit TF2 in T2CON. This bit can then be used to generate an interrupt. If EXEN2 = 1, Timer 2 performs the same operation, but a l- to-0 transition at external input T2EX also causes the cur- rent 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 bit, like TF2, can generate an interrupt. The capture mode is illus- trated in Figure 1. Auto-Reload (Up or Down Counter) 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 9). 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. Figure 2 shows Timer 2 automatically counting up when DCEN = 0. In this mode, two options are selected by bit Table 8. Timer 2 Operating Modes RCLK + TCLK CP/RL2 TR2 MODE 0 0 1 16-bit Auto-Reload 0 1 1 16-bit Capture 1 X 1 Baud Rate Generator X X 0 (Off) Figure 1. Timer 2 in Capture Mode OSC EXF2 T2EX PIN T2 PIN TR2 EXEN2 C/T2 = 0 C/T2 = 1 CONTROL CAPTURE OVERFLOW CONTROL TRANSITION DETECTOR TIMER 2 INTERRUPT ÷12 RCAP2L RCAP2H TH2 TL2 TF2 |
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