Firmware system design for a frequency and time interval analyzer - technical

Hewlett-Packard Journal, Feb, 1989 by Terrance K. Nimori, Lisa B. Stambaugh

Several display procedures are developed to simplify the interface to the CRT driver circuitry. These procedures access the tree types of display primitives, which are the building blocks of a screen: moves, lines, and text. These procedures provide the ability to:

* Clear the display

* Move the graphics "pen" to a specified pixel

* Draw a line from the current position of the graphics pen to another position

* Display a label

* Select the inverse video mode

* Control the intensity of a pixel.

The display procedures are patterned after the graphics commands of the HP 9000 Series 200/300 Computers. Their similarities made it possible to simulate a portion of the menu and graphics firmware on an HP 9000 Model 236 Computer before any instrument hardware was developed.

Menu Structure. An important design goal was a front panel interface tailored to the measurement technology while conforming to conventions established by previous counters. This goal implied the following guidelines:

* Instrument parameters should be visible at a glance, similar to the front panel of a conventional counter

* Parameters should be organized in a sequential and/or hierarchical structure to clarify their functions or interactions

* Menu levels should be minimized to provide direct access to parameters

* Prompts and help screens should be provided to reinforce user actions and clarify concepts.

These guidelines were used to evaluate softkey-driven and cursor-driven menu structures. A softkey-driven menu structure provides menu options at each lower level that specifically relate to options selected at previous levels. By guiding the user through options based on previous choices, it progressively converges to a limited set of options. This hierarchical structure can be perceived as a major drawback, however, because it restricts the range of user control at successive menu levels, and may require a considerable amount of menu traversal.

In contrast, a cursor-driven menu structure maintains a wide range of user control by minimizing menu levels. Related menu options are simultaneously displayed on a menu page, rather than on different menu levels. An option is modified by positioning the menu cursor to its corresponding data field and using softkeys to select other choices. Compared to the softkey-driven structure, it is loosely directed and often requires a large portion of the display area.

The cursor-driven structure was chosen primarily because it supports more descriptive menu screens. Cursor-driven menus are well-suited to describing instrument settings and showing hierarchical or chronological relationships between them. This was particularly helpful in presenting the sophisticated measurement arming capabilities of the HP 5371A (Fig. 3). Another benefit is that parameter interactions are more clearly visible. For example, conflicts between the meassurement function and the arming mode can be seen without traversing through a series of softkey choices.

Parameter interdependencies also influenced the softkey structure. In the HP 5371A, valid choices for the selected parameter are mapped to softkeys. Pressing one softkey repetitively to cycle through a set of options would have been simpler to implement, but might force the user to cycle through an intermediate option that is coupled to another parameter. By permitting direct selection of each option, we did not have to be concerned about inadvertent changes to instrument settings that might result from this process.