December 18, 1997

Controller provides multimode phase control

William Grill, Riverhead Systems, Littleton, CO

Using an inexpensive 16C508 multichip controller (Figure 1), you can implement a phase controller that provides a two-key, indexed processor for ac power or other discrete, digital-signal applications. The controller provides the switch-debounce function, single stepping and autostepping, index listing, and synchronization to edge transitions of the monitored ac signal or digital source. Four modes provide flexible support in several applications. You can configure Pin 3 (sense) and Pin 4 (mode) to provide either a 50-µsec pulse or a static output, with either a high or low true-output sense (Figure 2). In pulsed mode, indexing sequences from maximum to minimum; in static mode, index steps range from the minimum to maximum delay, as coded in the µC's index table.

The two indexing inputs on pins 7 and 8 increment or decrement an internal index counter and schedule an evaluation of the positive duty cycle of input 2 whenever a new step index occurs. The µC multiplies this local timing, corresponding to 180º, by the 8-bit value in an internally coded table, indexed by the internal-counter value. The design uses the most significant 24 bits of the result to provide the count for the equalized-delay process. When enabled, the equalized-delay process initiates upon detection of any transition at the pin-2 input. Use of an index table allows scheduling of both linear-phase or other, nonlinear tabled delays to the output. The controller's delay and key-detection loops limit operation to approximately 50-µsec steps, with memory space for as many as 200 index-step values.

For 60-Hz operation, you can define [(1/60)(180/360)/] 0.00005, or 166 unique index steps. The sizes of the internal registers and counters allow frequency operation from below 1 Hz to a practical limit of more than 1 kHz. By exploiting the 16C508's internal 4-MHz RC-based oscillator and configurable pullups, you need no external components. As configured, the coded index table provides linear steps of 5º increments with an initialized index count of 0. The circuit interprets processed delay-table values of zero as corresponding to an output disable. Indexing delays are code-settable; in this configuration, they are set for approximately 5-msec debounce and 0.1-sec autoindexing delay. Click here to download the hex file and µC code listing for the controller from DI-SIG, #2124. (DI #2124)

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