OM6211 NXP Semiconductors, OM6211 Datasheet - Page 42

• Manufacturer Part Number: OM6211
• Description: 48 X 84 Dot Matrix Lcd Driver
• Manufacturer: NXP Semiconductors

Philips Semiconductors
22 APPLICATION NOTES
When reading the VM bit in the OM6211 two problems
have been observed: corrupted format and VM bit
toggling.
22.1
The read-out of the VM bit has a special format, 11110000
for VM = 1 and 00001111 for VM = 0. However,
sometimes a wrong format of the read-out byte can be
observed; the first or the fifth or the eighth bit appears to
be wrong. There are two reasons for this behaviour. When
the first bit happens to be read out at the end of a frame
then it is possible that the first bit belongs to the old VM
value and the 7 following bits belong to the new VM value.
Such behaviour is possible for the first bit only. The second
reason is the violation of the OM6211 timing, if the timing
parameters t
results in reading a wrong value for the first, the fifth or the
eighth bit. Thus, to prevent any problems with the wrong
format of the read-out byte, these bits should always be
ignored.
22.2
Under certain conditions it can happen that the result of
reading VM is 0 even if the generated V
correct (VM bit toggles). It is therefore recommended to
repeat the VM read command several times according to
the algorithm described below. This algorithm is based on
the observation that a single reading of VM = 1 (after
numerous readings of VM = 0) is enough to ensure that
the charge pump operation is correct. One possible
method which gives minimum measurement duration is
shown in Fig.31 and described in detail below:
2002 Jan 17
Perform initialization with Enable OTP and set the
operational parameters (V
TC = 1, E = 0 and MY = 0) this results in a slightly higher
V
at T
Select DAL = 1 and DON = 1 (for all pixels on)
After setting HVE = 1 start a loop of a continuous VM
reading (for example, every 1 ms), at first occurrence of
reading VM = 1 interrupt the loop and accept VM = 1
When the reading is always VM = 0, stop the loop after
a certain time and accept VM = 0. This loop time limit
should be chosen sufficiently long, e.g. 85 ms. Given
that the uncertainty is much less then 0.1%, much less
then 1 ppm is expected to be read out wrong.
48
LCD
amb
Corrupted format
VM bit toggling
voltage than for normal operation (V
84 dot matrix LCD driver
= 27 C)
2
and t
3
(see Fig.17) are violated, then it
PR
= 159, S = 10, BS = 101,
LCD
LCD
voltage is
= 8.732 V
42
For the loop time limit a value of not less than 85 ms is
suggested. It should be noted that the value of 45 ms
specified in Section 11.2.2 means that after at least 45 ms
the VM measurement is possible. In practice it can be
expected that VM is valid earlier than 45 ms. Therefore the
proposed algorithm results in an optimization of the 45 ms
wait time needed to charge the external V
So the selected loop time limit of 85 ms consists of 45 ms
wait time and an additional 40 ms of measurement time.
The loop time limit of 85 ms will ensure that even if the first
VM = 1 value for any reason should be missed, there is
always the possibility to hit the next VM = 1 value (note
that the internal VM measurement is made once per
12.5 ms). However, the expectation is that the average
running time of the loop will be less than 45 ms.
There is another possibility for optimization: during the wait
time of the loop (1 ms) other tasks can be performed.
Furthermore the first part of the 45 ms wait time, just after
setting HVE = 1, may also be used for other tasks. For
instance when the first 20 ms are reserved for those tasks,
then the corresponding loop time limit would be 65 ms and
the expected average loop time would be less than 25 ms.
After the VM test is completed the V
desired value (e.g. V
being switched off.
PR
= 137) without the charge pump
PR
Product specification
can be set to the
LCD
OM6211
capacitor.