ST7920
Function Description
System interface
ST7920 supports 3 kinds of bus interface to communicate with MPU: 8-bit parallel, 4-bit parallel and clock
synchronized serial interface. Parallel interface is selected by PSB=”1” and serial interface is by PSB=”0”. 8-bit / 4-bit
interface is selected by function set instruction DL bit.
Two 8-bit registers (Data Register DR and Instruction Register IR) are used in ST7920 to access DRAM or Register.
Data Register (DR) can access DDRAM, CGRAM and GDRAM through the address pointer implemented by Address
Counter (AC). Instruction Register (IR) stores the instruction sent by MPU to ST7920.
4 kinds of parallel interface access mode can be selected through RS and RW:
RS
RW
Description
L
L
L
H
L
MPU write instruction to instruction register (IR)
MPU read busy flag (BF) and address counter (AC)
MPU write data to data register (DR)
H
H
H
MPU read data from data register (DR)
* The serial interface access modes do not have Read operation.
Busy Flag (BF)
ST7920 needs a process time for any received instruction. Before finishing the received instruction, any further
instruction is not accepted. The process time of each instruction is not equal and the internal process is finished or
not can be determined by the BF. Internal operation is in progress while BF=”1”, that means ST7920 is in busy state.
No further instructions will be accepted until BF=”0”. MPU must check BF to determine whether the internal operation
is finished or not before issuing instruction.
Address Counter (AC)
Address Counter (AC) is used as the address pointer of DDRAM, CGRAM and GDRAM. (AC) can be set by
instruction. After that, accesses (Read/Write operations) to the memories, such as DDRAM, CGRAM or GDRAM,
(AC) will be increased or decreased by 1 (according to the setting in “Entry Mode Set” Register). When RS=”0”,
RW=”1” and E=”1” the value of (AC) will be output to DB6~DB0.
Character Generation ROM (CGROM) and Half-width Character Generation ROM (HCGROM)
ST7920 is built in a Character Generation ROM (CGROM) to provide 8192 16x16 character fonts and a Half-width
Character Generation ROM to provide 126 8x16 alphanumeric characters. It is easy to support multi-language
applications such as Chinese and English. Two consecutive bytes are used to specify one 16x16 character or two
8x16 half-width characters. Character codes are written into DDRAM and the corresponding fonts are mapped from
CGROM or HCGROM to the display drivers.
Character Generation RAM (CGRAM)
ST7920 is built in a Character Generation RAM (CGRAM) to support user-defined fonts. Four sets of 16x16
bit-maped RAM spaces are available. These user-defined fonts are displayed the same ways as CGROM fonts by
writing the related character code into the DDRAM.
V4.0
10/49
2008/08/18
SITRONIX [ SITRONIX TECHNOLOGY CO., LTD. ]
