7220 Datasheet, PDF (181/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPLICATION
5.1 Example of multi-line display
5.1.6 Processing time
When setting display data by a CRT interrupt, the processing time is limited.
As shown in Figure 5.1.7, a CRT interrupt occurs at the end of the first line (block 1) display and setting
for the 3rd line display is started. This setting must be completed before a scanning line reaches to the
3rd line display position. If the setting is not completed, display characters flicker or rewriting is looked.
And also, for multi-line display of 12 lines, be sure that CRT interrupts occur 12 times from a VSYNC to the
next VSYNC. If CRT interrupts do not occur as many times as the number of display lines, the following
causes can be assumed.
• Display position overlaps.
• CRT interrupt processing time is too long, resulting in no display of that line (2 lines after the line being
displayed).
For example, a CRT interrupt occurs at the end of the second line display in Figure 5.1.7. Within this
interrupt processing, setting for the 4th line display is completed. However, if a scanning line is over the
display position of the 4th line (that is, A in Figure 5.1.7) during this setting, one CRT interrupt request is
deleted (or does not occur). Therefore, the line counter value is disordered and multi-line display is not
displayed correctly. In such cases, due to whatever causes, correct the value with processing C of VSYNC
interrupt processing (refer to “Figure 5.1.4”). When the CRT interrupt software processing overtime causes
this state, change the display positions or shorten the CRT interrupt software processing time.
5-8
7220 Group User’s Manual