ATMEGA640 datasheet(141/436 Pages) MICROCHIP

141 / 436 page

141

ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

17.6.1

Input Capture Trigger Source

The main trigger source for the input capture unit is the Input Capture Pin (ICPn). Timer/Counter1 can alternatively

use the analog comparator output as trigger source for the input capture unit. The Analog Comparator is selected

as trigger source by setting the analog Comparator Input Capture (ACIC) bit in the Analog Comparator Control and

Status Register (ACSR). Be aware that changing trigger source can trigger a capture. The input capture flag must

therefore be cleared after the change.

Both the Input Capture Pin (ICPn) and the Analog Comparator output (ACO) inputs are sampled using the same

technique as for the Tn pin (Figure 18-1 on page 164). The edge detector is also identical. However, when the

noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by four

system clock cycles. Note that the input of the noise canceler and edge detector is always enabled unless the

Timer/Counter is set in a Waveform Generation mode that uses ICRn to define TOP.

An input capture can be triggered by software by controlling the port of the ICPn pin.

17.6.2

Noise Canceler

The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input is

monitored over four samples, and all four must be equal for changing the output that in turn is used by the edge

detector.

The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter Control

from a change applied to the input, to the update of the ICRn Register. The noise canceler uses the system clock

and is therefore not affected by the prescaler.

17.6.3

Using the Input Capture Unit

The main challenge when using the Input Capture unit is to assign enough processor capacity for handling the

incoming events. The time between two events is critical. If the processor has not read the captured value in the

ICRn Register before the next event occurs, the ICRn will be overwritten with a new value. In this case the result of

the capture will be incorrect.

When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler routine

as possible. Even though the Input Capture interrupt has relatively high priority, the maximum interrupt response

time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt requests.

Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively changed during

operation, is not recommended.

Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each capture.

Changing the edge sensing must be done as early as possible after the ICRn Register has been read. After a

change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a logical one to the I/O bit

location). For measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt handler is

used).

17.7 Output Compare Units

The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT equals

OCRnx the comparator signals a match. A match will set the Output Compare Flag (OCFnx) at the next timer clock

cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an Output Compare interrupt. The OCFnx Flag

is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag can be cleared by software

by writing a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output

according to operating mode set by the Waveform Generation mode (WGMn3:0) bits and Compare Output mode

(COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform Generator for handling the special

cases of the extreme values in some modes of operation. See “Modes of Operation” on page 144.

ATMEGA640 датащи(PDF) 141 Page - Microchip Technology

ATMEGA640 датащи(HTML) 141 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL