On-Screen DisplayVRAMMN102H75K/F75K/85K/F85K LSI User Manual Panasonic Semiconductor Development Company171Panasonic7.8.3 Cautions about the number of display code set to VRAMWhen the display lines are adjoined or overlapped, and the number of the abovedisplay code is extremely fewer than that of the below one, first line of thedisplay line may not be output correctly.In OSD circuit, font data to display are read from ROM and stored to the buffer.The data in the buffer are overwritten, after the present display code is output, tothe display data of the next line.Show the example when the display lines are adjoined. The number of thedisplay code in the above line (line M) is reffered to n, and that of the below one(line M+1) is refferred N.The buffer holds n display data of the last line of line M at the time of A (seefigure below), and the data in the buffer are overwritten to the display data of thefirst line of line M+1 as time goes by. At the time of B the buffer holds n displaydata of the first line of line M+1.In case n is smaller than N, N-n display data are needed to display the first line ofline M+1, and to be written to the buffer before it is shown on the display.Especially when n is extremely smaller than N, a large amount of the display datais needed to be written to the buffer before display. And in case writing is notdone in time, it shows wrong data on the screen.To prevent the wrong display written above, the first line of the below line is tobe shown later enough after the last line of the above line is displayed.The time (Td) needed for the correct display is caluculated as below.Td = 0.8 + ( N - n ) × 0.5 [μs]n : the number of display code in the above line. (include HP, VP)N: the number of display code in the below line. (include HP, VP)nFigure 7-8 Timing for OSD dataHSYNCdisplayBufferCBA
