PCF8535 datasheet(12/52 Pages) PHILIPS

12 / 52 page

1999 Aug 24

Philips Semiconductors

Objective speciﬁcation

× 133 pixel matrix driver

PCF8535

Table 4

Example of LCD bias voltage for 1/7bias, n = 3

7.11

Temperature measurement

7.11.1

TEMPERATURE READ BACK

The PCF8535 has an in-built temperature sensor.

For power saving, the sensor should only be enabled

when a measurement is required. It will not operate in

power-down mode. The temperature read back requires a

clock to operate. Normally the internal clock is used but, if

the device is operating from an external clock, then this

must be present for the measurement to work. VDD2 and

VDD3 must also be applied. A measurement is initialized by

setting the SM bit. Once started the SM bit will be

automatically cleared. An internal oscillator will be

initialized and allowed to warm-up for approximately

2 frame periods. After this the measurement starts and

lasts for a maximum of 2 frame periods.

Temperature data is returned via a status register. During

the measurement the register will contain zero. Once the

measurement is completed the register will be updated

with the current temperature (non zero value). Because

the I2C-bus interface is asynchronous to the temperature

measurement, read back prior to the end of the

measurement is not guaranteed. If this mode is required

the register should be read twice to validate the data.

The ideal temperature read-out can be calculated as

follows;

(1)

where T is the on-chip temperature in

°C and c is the

conversion constant; c = 1.17

°C/lsb.

To improve the accuracy of the temperature measurement

a calibration is recommended during the assembly of the

final product.

For calibrating the temperature read-out a measurement

must be taken at a defined temperature. The offset

between the ideal read-out and the actual result has to be

stored into a non-volatile register (e.g. EEPROM);

(2)

where TRmeas is the actual temperature read-out of the

PCF8535.

The calibrated temperature read-out can be calculated for

each measurement as follows:

(3)

The accuracy after the calibration is

±6.7% (plus ±1 lsb) of

the difference between the current temperature and the

calibration temperature. For this reason a calibration at or

near the most sensitive temperature for the display is

recommended.

E.g. for a calibration at 25

°C with the current temperature

−20 °C, the absolute error may be calculated as:

Absolute error = 0.067

× (25 °C −−20 °C)

±3 °C+ ±1 lsb = ±4.17 °C.

7.12

Temperature compensation

7.12.1

TEMPERATURE COEFFICIENTS

Due to the temperature dependency of the liquid crystals

viscosity the LCD controlling voltage, V must be increased

at lower temperatures to maintain optimum contrast.

Figure 4 shows VLCD as a function of temperature for a

typical high multiplex rate liquid.

In the PCF8535 the temperature coefficient of VLCD can be

selected from 8 values by setting bits TC[2:0],

see Table 5.

SYMBOL

BIAS VOLTAGE FOR 1/7BIAS

VLCD

6/7 × VLCD

5/7 × VLCD

2/7 × VLCD

1/7 × VLCD

VSS

ideal

128

°C

–

()

---

Offset

ideal

meas

–

cal

meas

Offset

PCF8535 Datasheet(PDF) 12 Page - NXP Semiconductors

PCF8535 Datasheet(HTML) 12 Page - NXP Semiconductors

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Datasheet Download

Link URL