
7 HARDWARE ROTATION
C-2-26
EPSON
SID13705 PROGRAMMING NOTES
AND EXAMPLES
7.3 Alternate Portrait Mode
Alternate portrait mode does not impose the power of two line width. To rotated the image on 240
line panel requires a portrait stride of 240 pixels. Alternate portrait mode is capable of scrolling by
one line at a time in response to changes to the Start Address Registers. However, to achieve the
same frame rate requires a 2 x faster input clock, therefore using more power.
The following figure depicts the ways to envision memory layouts for the SID13705 block in
alternate portrait mode. This example also uses a 480
× 320 panel. Notice that in alternate portrait
mode the stride may be as little as 240 pixels.
Figure 7-2 Relationship Between the Alternate Mode Screen Image and the Image Refreshed by SID13705 block
From the programmers perspective the memory is laid out as shown on the left. The programmer
accesses memory exactly as for a panel of with the dimensions of 480
× 320. The programmer sees
memory addresses increasing from A->B and from C->D.
From a hardware perspective the SID13705 block always refreshes the LCD panel in the order B-
>D and down to do A->C
The greatest factor in selecting alternate portrait mode over default portrait mode would be for the
ability to obtain an area of contiguous off screen memory. For example: A 640
× 480 panel in
default portrait mode at one bit-per-pixel requires 40960 bytes (40 Kb). There is unused memory
but it is not contiguous. The same situation using alternate portrait mode requires 38400 bytes
leaving 2560 bytes of contiguous memory available to the application. In fact the change in
memory usage may make the difference between being able to run certain panels in portrait mode
or not being able to do so.
image seen by programmer
= image in display buffer
480
portrait
window
480
320
AB
C
D
C
B
A
320
start
address
por
tr
ait
window
display
image refreshed by SID13705 block
start
address
physical
memory