How to use CPLDs to implement a QWERTY keypad

By Mike Gulotta, Xilinx
November 30, 2006 -- pldesignline.com

This tutorial describes how low cost, low power CPLDs can be used to expand a typical handset DTMF keypad into a QWERTY keypad.

As cell phones and other portable handheld devices continue to add features, design trade-offs are constantly evolving. Popular features such as text messaging and web browsing demand more data entry, but that can be cumbersome with traditional dual-tone multiple frequency (DTMF) (0-9, #, *) keypads. Using this type of keypad requires multi-tap data entry, which is inefficient and error-prone.

One option for making text entry easier is to use a QWERTY keypad (Fig 1). This type of keypad employs 40 or more keys versus the normal 12 in a DTMF handset. Although the additional keys make the handset larger and involve more electronic components, text message users may be willing to trade size for a QWERTY keypad; text entry is much easier and you can use two thumbs to enter text messages or data. Recently, some cell phone manufacturers have released handsets with QWERTY keypads that cater to text users.


1. QWERTY keypad (Motorola model A630).

There are many ways to design data entry keypads, but no standard exists. In this article, we'll examine a low power, low cost CPLD with small package options as one possible solution for addressing the design challenge of adding keys to a traditional DTMF-type keypad.

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