Rainbow Electronics HD44780 - Manual (Page 22)

HD44780U
22
Interfacing to the MPU
The HD44780U can send data in either two 4-bit operations or one 8-bit operation, thus allowing
interfacing with 4- or 8-bit MPUs.
•
For 4-bit interface data, only four bus lines (DB4 to DB7) are used for transfer. Bus lines DB0 to DB3
are disabled. The data transfer between the HD44780U and the MPU is completed after the 4-bit data
has been transferred twice. As for the order of data transfer, the four high order bits (for 8-bit operation,
DB4 to DB7) are transferred before the four low order bits (for 8-bit operation, DB0 to DB3).
The busy flag must be checked (one instruction) after the 4-bit data has been transferred twice. Two
more 4-bit operations then transfer the busy flag and address counter data.
•
For 8-bit interface data, all eight bus lines (DB0 to DB7) are used.
RS
R/W
E
IR7
IR6
IR5
IR4
BF
AC6
AC5
AC4
DB7
DB6
DB5
DB4
Instruction register (IR)
write Busy flag (BF) and
address counter (AC)
read Data register (DR)
read
IR3
IR2
IR1
IR0
AC3
AC2
AC1
AC0
DR7
DR6
DR5
DR4
DR3
DR2
DR1
DR0
Figure 9 4-Bit Transfer Example