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TMR0 DELAYS
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One of the basic operations of a timer is to provide a set time delay. In many of my experiments and applications, I often use TMR0 to request an interrupt after a speci c time delay. It isn t dif cult to use a timer to calculate a delay, although there are a few things you should be aware of before you attempt it. The rst thing that you should recognize is that the interrupt is requested when TMR0 over ows or equals 256. 255 or 0xFF is the largest value that can be saved in the bit timer register. 0xFF does not cause an interrupt (although an interrupt will be requested when the timer increments or over ows to 256 or 0x100). Therefore, the delay is calculated for the time when the timer equals 256 and not 255. The initial timer value must be the number of clock increments (or ticks) required to get to 256 within a speci c period of time because the timer can only count up. To calculate the initial value (with no prescaler, which will be described later), the following formula is used: TMR0 initial 256 delay cycles
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The delay cycles value is found by taking the delay time, dividing by the frequency, and multiplying by 4: Delay cycles (delay time * frequency)/4
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If the prescaler is speci ed for TMR0, and even if it is left at 1:1, then each clock cycle is divided by 2 before it is passed to TMR0. This changes the formula for delay cycles to Delay cycles [(delay time * frequency)/4]/2 (delay time * frequency)/8
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These formulas can be used to calculate a 160- s delay in a 3.58-MHz PIC microcontroller application. Assuming that the prescaler is not assigned to TMR0, the rst formula for delay cycles can be used, and the number of delay cycles timed by is calculated as Delay cycles (delay time * frequency)/4 (160 s * 3.58 MHz)/4 [160(10 6) s * 3.58(106) 1/s]/4 143.2 143
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(160 * 3.58)/4
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The result is rounded to the nearest whole number so that it can be stored in TMR0. This value is used to calculate the initial timer value using the rst formula Initial 256 delay cycles 256 143 113
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Therefore, to wait 160 s before a TMR0 over ow interrupt in a PIC microcontroller running at 3.58 MHz, an initial value of 113 must be loaded into TMR0. For longer delays than 256 instruction cycles, the prescaler can be used to divide the number of cycles input into the timer. The prescaler, as will be discussed in the next section, divides the incoming data by powers of 2 from 2 to 256. When calculating the delay, the delay cycles are halved continually until the value is less than 256. For example, if a delay of 5 s were required using the original formula in a PIC microcontroller running at 4 MHz, the delay cycles would be calculated as Delay cycles (delay time * frequency)/4 (5 ms * 4 MHz)/4 [5(10 3) s * 4(106) 1/s]/4 5(103) 5,000
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Since the calculated delay cycle is greater than 256, it is continually divided by 2 to get an appropriate prescaler divisor. Dividing by 2, the delay count would be 2,500, which is still greater than 256. Dividing by 2 again, delay count would be 1,250, which is also greater than 256. Dividing the original value by 8 yields a delay count of 625, which is still greater than 256, as does dividing by 16 (which results in 312.5). Finally, dividing the original by 32 yields a delay count of 156.25. Rounding it off, the value can be put into the initial value formula: Initial 256 156 100
Now, the rounding that I did (taking off 0.25) will result in a difference of four cycles between the actual delay of 5 ms and the delay calculated within TMR0. If this is a critical loss, then I suggest that you do one of two things. The rst is to add four delay cycles to your code, which can be four nops, a dummy call, or even delaying loading the timer by four instruction cycles.
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