hd66421 Renesas Electronics Corporation., hd66421 Datasheet - Page 22

• Manufacturer Part Number: hd66421
• Description: Ram-provided 160 Channel 4-level Grey Scale Driver For Dot Matrix Graphics Lcd
• Manufacturer: Renesas Electronics Corporation.

Access to Internal Registers and Display RAM
Access to Internal Registers by the MPU: The
internal registers includes the index register and
data registers. The index register can be accessed
by driving both the CS and RS signals low. To
access a data register, first write its register
number ID to the index register with RS set to 0,
and then access the data register with RS set to 1 .
Once written, the register number is held until it
is rewritten, enabling the same register to be
consecutively accessed without having to rewrite
to the register number for each access. Some data
registers contain unused bits; they should be set
to 0. Note that all data registers except the display
memory access register can only be written to.
Access to Display RAM by the MPU: To access
the display RAM, first write the RAM address
desired to the X address register (R2) and the Y
address register (R3). Then read/write the display
memory access register (R4). Memory access by
the MPU is independent of memory read
by the HD66421 and is also asynchronous with
the HD66421's clock, thus enabling an interface
independent of HD66421's internal operations.
Address
Input
data
Output
data
WR
RD
RS
Preliminary
H'02
[*,*]
X Address
[n]
Figure 16 Display RAM read sequence
H'03
[n,*]
Y Address
[m]
22
However, when reading. data is temporarily
latched into a H66421's buffer and then output
next time, a read is performed in a subsequent
cycle. This means that a dummy read is necessary
after setting X and Y addresses. The memory
read sequence is shown in figure 16.
X and Y addresses are automatically incremented
after each memory access according to the INC
bit value in control register 2; therefore, it is not
necessary to update the addresses for each access.
Figure 16 shows two cases of incrementing
display RAM address. When the INC bit is 0, the
Y address will be incremented up to H'7F with
the X address unchanged. However, actual
memory is valid only within H'00 to H'4F;
accessing an invalid address is ignored. When the
INC bit is 1 , the X address will be incremented
up to H'27 or H'35 according to WLS bit with the
Y address unchanged. After address H'27 or
H'35, the X address will be returned to H'00;
accessing more than forty bytes causes rewriting
to the same address.
H'04
[n,m]
Undetermined
Dummy
read
Data[n,m]
[n,m+1]
HD66421
Data[n,m+1]
[n,m+2]