{"id":10522,"date":"2015-12-05T05:52:39","date_gmt":"2015-12-05T10:52:39","guid":{"rendered":"http:\/\/embedded-lab.com\/blog\/?p=10522"},"modified":"2016-12-18T19:43:31","modified_gmt":"2016-12-19T00:43:31","slug":"xmega-analog-comparator","status":"publish","type":"post","link":"https:\/\/embedded-lab.com\/blog\/xmega-analog-comparator\/","title":{"rendered":"XMega Analog Comparator"},"content":{"rendered":"

Previously we dealt with the XMega Analog-to-Digital Converter (ADC)<\/i><\/b><\/a> block. We know that we can use the ADC to measure voltages and take decisions based on voltage values\/levels but sometimes it is enough to detect voltage levels and not to measure the exact voltage values. In such occasions where we just need to check voltage levels relative to a reference or threshold value, we need an Analog Comparator (AC)<\/i><\/b>. An analog comparator can be used to compare two voltage levels and based on that it can be used to generate a logic output (0 or 1<\/i>) to indicate which of the two levels is higher or lower than the other. That\u2019s all and there isn\u2019t much about analog comparators. The XMega family of micros come loaded with high performance dual analog comparator modules. However so far we saw that between the traditional 8-bit micros and the XMega micros, the major difference apart from programming is the overall nifty enhancements in all common hardware blocks. When it comes to the analog comparators of the XMega micros, the same is true. In this issue we will explore the XMega analog comparator block.<\/p>\n

\"Block<\/a><\/p>\n

 <\/p>\n

<\/p>\n

A Brief Overview<\/h4>\n

Generally a comparator is a device that compares two quantities. Analog comparators compare voltage \u2013 an analog quantity. An analog comparator is essentially an op-amp with infinite gain. It has one output and two inputs. The output is switched to VDD<\/i> or ground (0V) depending on the relative voltage levels in the two input pins. Just as with an op-amp the two inputs are labeled as inverting input (INV<\/i>) and non-inverting input (NINV<\/i>). When the non-inverting input voltage is higher than that of the inverting input, the output switches to VDD<\/i> and when it is the other way, the output switches to ground. Thus we get two logic states.<\/p>\n

\"Formula\"<\/a><\/p>\n

 <\/p>\n

In the XMega micros, the basic comparator that one may find in a typical 8-bit micro is enhanced a bit. The basic operations are same but additionally a few features have been included. For instance we can add hysteresis and can change operational speed of these comparators by altering propagation delays. Hysteresis is useful feedback mechanism for fast varying input signals (especially ringing\/oscillating signals) that cause high rates of output swings. By adding hysteresis to a comparator we can effectively reduce the frequent oscillation of the output.<\/p>\n

\"Comparator<\/a>\"Hysteresis\"<\/a><\/p>\n

 <\/p>\n

In the ATXMega micros, two levels of hysteresis can be programmed \u2013 small (30mV<\/i>) and large (60mV<\/i>). The datasheet documents of the XMega devices state these values and much more. As the name suggests High Speed mode yields in faster operation while the default Low Power mode is slower than the former but it is much more energy efficient. Shown below is the table for electrical specs of the ATXMega32A4U\u2019s analog comparator module. Check it out:<\/p>\n

\"Electrical<\/a><\/p>\n

Unlike old school micros, the input pins to the comparators are not tied to a specific set of I\/O pins of an I\/O port. We can literally use any pin of a port that is related to the comparator block. I have used an ATXMega32A4U micro for this issue. This micro has a pair of analog comparators under a single comparator block called Analog Comparator A (ACA)<\/i><\/b>. Larger XMega devices have more of such comparator blocks. Each block has a pair of comparator modules. Each comparator in a block has a unique designation like ACA.AC0<\/i><\/b>, meaning the first analog comparator of block A comparator pair attached with PORTA. I\/Os related to ACA are available to PORTA pins only. Watch the blue boxes in the image below:<\/p>\n

\"XMega-A4-Pin-Map\"<\/a><\/p>\n

We can also set a pair of comparators as a window comparator. Window comparators are also known as limit comparators because of their limit sensing ability. Such comparators determine when the input voltage to it is within a preselected range. There are two voltage limit values (upper limit and lower limit) or references for an input signal that define the boundaries of the voltage window. An input can be within, outside, above or below the predefined window and so there are four states.<\/p>\n

\"Window<\/a><\/p>\n

Window comparators have several applications. One good application is a battery charger. A window comparator will always monitor the voltage level of a battery with respect to references and based on the four aforementioned output conditions, charging can be started or stopped automatically without needing much coding and attention.<\/p>\n

Three internal reference sources can be tied with the comparator pins. These are as follows:<\/p>\n