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Embedded Systems December 2000 Vol13_13

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spond to a different phase or opera- tion. In such cases the modes wi ll seem natural and n.ecessary to the user. I expect my calculator to behave differ- enuy in hexadecimal mode than in decimal mode. Modes have a bad reputation in the us r interface community, and ulis reputation has been earned by many user interfaces that make terrible use of modes. Text editors like vi , with insert and control modes, can cause havoc when the use r starts to type a sentence, only to realize that he has just typed seven different commands, and half of th e screen has been Theirs indented and changed to upper case. My microwave oven has a gri ll mode and a microwave mode. I have often walked up to it, inse rted some food, and turned it on for live minutes, only to realize that T had the oven in gri ll mode rather than microwave mode. The problem is not \vith the modes themselves, which are often quite nec- essary, but with recognizing which mode the device is in. Sometimes the clue is quite small and easy to over- look. My VCR has a play mode and a record mode. There is never any con- fusion, because the word PLAY or RECORD appears on a large one-l ine text display when one of those modes is active. So little other information is being presented that I cannot over- look ulis visual clue. The most troubl esome modes Microcontroller analog emulation. They measure analog after going through 10" of wire. often are invented for the sole pur- pose of allowing some of th e conu'ols or displays to be multiplexed. This is true for the text editor, which would be far more useful if more buttons were available that were dedicated to the command functions. If you are short of space for extra con trois or dis- plays, you may not be able to justify much space for a mode indicator. Now you have two bad features: a mode ulat does not refl ect a different task or role for the user, and a mode that does not make itself obvious. Use modes where necessary, but always make the current mode obvious. Equal opportunity Keeping the paths Ulat ule user has to follow short and simple is always an advantage. One of the ways to keep the paths short is to use ule data already available from the device out- put. When you select cruise control in a car, you do not have to dial in the speed, since it is already availabl e. The interface takes one of the outputs, the current speed as displayed on the speedometer, and uses it as an input to the cruise control system. A VCR may use a similar mechanism. If the u er is setting a timel', the currently selected chan nel can be assumed a the chan- 66 DECEMBER 2000 Embedded Systel11s Programming

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