PCF8820 datasheet(26/60 Pages) PHILIPS

26 / 60 page

2000 Dec 07

Philips Semiconductors

Product speciﬁcation

× 101 Grey-scale/ECB colour dot matrix

LCD driver

PCF8820

It is not possible to measure temperature in Power-down

mode.

During a temperature measurement, the status register

value remains zero until the measurement has completed

and then the register is updated with the current

temperature value (non-zero value). Because the I2C-bus

interface is asynchronous to the temperature

measurement, the value read from the status register

should be validated by reading the status register a few

times.

During a temperature measurement, the temperature

coefficient (TC) has to be selected.

The ideal temperature read-out can be calculated by the

equation:

where T is the on-chip temperature in

°C and ‘a’ is the

conversion constant (see Chapter 11).

To improve the accuracy of the temperature

measurement, it is recommended that the temperature

read-out is calibrated during the product’s final assembly.

Calibration of the temperature read-out requires a

measurement to be made at a defined ideal temperature.

The offset between the ideal temperature value and the

measured temperature value is calculated by:

TRoffset =TRideal − TRmeas

where TRmeas is the actual temperature read-out of the

PCF8820. The offset value must be stored in a non-volatile

A calibrated temperature read-out can be calculated for

each measurement by the equation:

TRcal =TRmeas +TRoffset

The accuracy after the calibration is

±10% ±1 bit of the

difference between the measured temperature and the

calibration temperature. For this reason, it is

recommended that a calibration is performed at or near the

most sensitive LCD temperature.

For example: calibration temperature is 25

°C and the

measured temperature is

−20 °C. The relative error

±0.10 × {25 − (−20)} ±1 bit × a

±4.5 ±1.13

±5.63 °C.

This calibration accuracy is valid for temperature

measurements made when the supply voltage value is the

same as when it was calibrated.

8.1.7

VLCD CONTROL REGISTER

The VLCDOUT value can be set by software using the

bits VOP6 to VOP0 of the VLCD control register.

The programmed value for VLCD has to be calculated for a

reference temperature, called the cut-point temperature

Tcp, using the equation:

VLCD (at Tcp)=a+b × VOP

The values for parameters Tcp, a and b are given in

Table 7, and their relationship with the VLCD control

The VLCDOUT generated is dependent on the operating

temperature Toper, the selected temperature coefficient TC

and the programmed value for VLCD at the reference

temperature Tcp and is calculated by the equation:

VLCD (at Toper)=VLCD (at Tcp) × {1 + TC × (Toper − Tcp)}

Two overlapping VLCD ranges are selectable by bit PRS

(see Table 7 and Fig.20). The maximum voltage that can

be generated depends on the values of VDD2 and VDD3,

and the display load current. At a multiplex rate of 1 : 67,

the optimum operating voltage for the LCD can be

calculated by the equation:

where Vth is the threshold voltage of the liquid crystal

material used.

The practical value for VLCD is determined by equating

Voff(rms) with the defined LCD threshold voltage (Vth), which

is the typically value when the LCD exhibits approximately

10% contrast.

Table 7

Parameter values for programming VLCD control

ideal

128

°C

–

()

---

SYMBOL

VALUE

UNIT

BIT PRS = 0

BIT PRS = 1

Tcp

23.0

°C

4.500

10.215

0.045

programming

range

4.5 to 10.215

10.215 to 15.93

LCD

167

--------------------

–





-------------------------------------------------

6.975

≅

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