OM4085 Philips Semiconductors, OM4085 Datasheet - Page 13

• Manufacturer Part Number: OM4085
• Description: Universal LCD driver for low multiplex rates
• Manufacturer: Philips Semiconductors

Philips Semiconductors
There is a one-to-one correspondence between the RAM
addresses and the segment outputs, and between the
individual bits of a RAM word and the backplane outputs.
The first RAM column corresponds to the 24 segments
operated with respect to backplane BP0 (see Fig.9).
In multiplexed LCD applications the segment data of the
second, third and fourth column of the display RAM are
time-multiplexed with BP1, BP2 and BP3 respectively.
When display data are transmitted to the OM4085 the
display bytes received are stored in the display RAM
according to the selected LCD drive mode. To illustrate the
filling order, an example of a 7-segment numeric display
showing all drive modes is given in Fig.10; the RAM filling
organization depicted applies equally to other LCD types.
With reference to Fig.10, in the static drive mode the eight
transmitted data bits are placed in bit 0 of eight successive
display RAM addresses. In the 1 : 2 multiplex drive mode
the eight transmitted data bits are placed in bits 0 and 1 of
four successive display RAM addresses. In the 1 : 3
multiplex drive mode these bits are placed in
bits 0, 1 and 2 of three successive addresses, with bit 2 of
the third address left unchanged. This last bit may, if
necessary, be controlled by an additional transfer to this
address but care should be taken to avoid overriding
adjacent data because full bytes are always transmitted.
In the 1 : 4 multiplex drive mode the eight transmitted data
bits are placed in bits 0, 1, 2 and 3 of two successive
display RAM addresses.
Data pointer
The addressing mechanism for the display RAM is
realized using the data pointer. This allows the loading of
an individual display data byte, or a series of display data
bytes, into any location of the display RAM.
1997 Feb 25
handbook, full pagewidth
Universal LCD driver for low multiplex
rates
Fig.9
Display RAM bit-map showing direct relationship between display RAM addresses and segment outputs,
and between bits in a RAM word and backplane outputs.
backplane outputs
display RAM bits
(columns) /
(BP)
0
1
2
3
0
1
2
display RAM addresses (rows)/segment outputs (S)
3
4
13
The sequence commences with the initialization of the
data pointer by the LOAD DATA POINTER command.
Following this, an arriving data byte is stored starting at the
display RAM address indicated by the data pointer thereby
observing the filling order shown in Fig.10. The data
pointer is automatically incremented according to the LCD
configuration chosen. That is, after each byte is stored, the
contents of the data pointer are incremented by eight
(static drive mode), by four (1 : 2 multiplex drive mode), by
three (1 : 3 multiplex drive mode) or by two (1 : 4 multiplex
drive mode).
Subaddress counter
The storage of display data is conditioned by the contents
of the subaddress counter. Storage is allowed to take
place only when the contents of the subaddress counter
agree with the hardware subaddress applied to
A0, A1 and A2 (pins 7, 8, and 9). A0, A1 and A2 should
be tied to V
defined by the DEVICE SELECT command. If the contents
of the subaddress counter and the hardware subaddress
do not agree then data storage is inhibited but the data
pointer is incremented as if data storage had taken place.
The subaddress counter is also incremented when the
data pointer overflows.
The storage arrangements described lead to extremely
efficient data loading in cascaded applications. When a
series of display bytes are being sent to the display RAM,
automatic wrap-over to the next OM4085 occurs when the
last RAM address is exceeded. Subaddressing across
device boundaries is successful even if the change to the
next device in the cascade occurs within a transmitted
character.
SS
or V
19
DD
. The subaddress counter value is
20
21
22
MGG389
23
Product specification
OM4085