The SPI Function - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 514

7 6 5 4 3 2 1 0

X X X X X X X Test

Test: test bit
0 = normal operation.
1 = display test.

Figure 10.29 ■ The MAX6952 display test register

and the duty cycle is 7/16 (half power). The contents of the display test register are shown in
Figure 10.29. Only bit zero of this register is implemented.

C HARACTER -G ENERATOR F ONT M APPING
The character font is a 5 3 7 matrix. The character generator comprises 104 characters in
ROM and 24 user-definable characters. The selection from a total of 128 characters is repre-
sented by the lower 7 bits of the 8-bit digit registers. The character map is in the Arial font for
96 characters in the range from 0x28 to 0x7F. The first 32 characters map the 24 user-defined
positions (RAM00 to RAM23), plus 8 extra common characters in ROM. When the most sig-
nificant bit is 0, the device will display the font normally. Otherwise, the chip will display the
font inversely.

U SER -D EFINED F ONT R EGISTER
The 24 user-definable characters are represented by 120 entries of 7-bit data, five entries
per character, and are stored in the MAX6952's internal RAM. The 120 user-definable font data
are written and read through a single register at the address 0x05. An auto-incrementing font
address register pointer in the MAX6952 indirectly accesses the font data. The font address
pointer can be written, setting one of 120 addresses between 0x00 and 0xF7, but cannot be
read back. The font data is written to and read from the MAX6952 indirectly, using this font
address pointer. Unused font locations can be used as general-purpose scratch RAM. Font reg-
isters are only 7 bits wide.
To define new fonts, the user first needs to set the font address pointer. This is done
by placing the address in the font address pointer register and setting bit 7 to 1. After this,
the user can write the font data in the lower 7 bits (to the font address pointer position) and
clear bit 7.
The font address pointer autoincrements after a valid access to the user-definable font data.
Auto-incrementing allows the 120 font data entries to be written and read back very quickly
because the font pointer address needs to be set only once. When the last data location, 0xF7,
is written into, the font address pointer increments to 0x80 automatically. If the font address
pointer is set to an out-of-range address by writing data in the range from 0xF8 to 0xFF, then the
address is set to 0x80 instead.
The memory mapping of user-defined font register 0x05 is detailed in Table 10.11. The
behavior of the font pointer address is shown in Table 10.12. To display the font defined by the
user, one must send in the RAM address from 0x00 through 0x17, corresponding to the font ad-
dress pointer value that is 5

× RAM address (one character needs 5 bytes).