CS5461A datasheet(20/44 Pages) CIRRUS

20 / 44 page

CS5461A

DS661F2

The Cycle Count Register (N) must be set to a value

greater than one. Status bit TUP in the Status Register,

indicates when the Temperature Register is updated.

The Temperature Offset Register sets the zero-degree

measurement. To improve temperature measurement

accuracy, the zero-degree offset should be adjusted af-

ter the CS5461A is initialized. Temperature offset cali-

bration is achieved by adjusting the Temperature Offset

sured from a calibrated digital thermometer and the

CS5461A temperature sensor. A one-degree adjust-

ment to the Temperature Register (T) is achieved by

adding 2.737649x10-4 to the Temperature Offset Regis-

ter (Toff). Therefore,

if Toff = -0.0951126 and ∆T= -2.0 (

oC), then

or 0xF3C168 (2’s compliment notation) is stored in the

Temperature Offset Register (Toff).

To convert the Temperature Register (T) from a Celsius

scale (oC) to a Fahrenheit scale (oF) utilize the formula

Applying the above relationship to the CS5461A tem-

perature measurement algorithm

If Toff = -0.09566 and Tgain = 23.507

for a Celsius

scale, then the modified values are Toff = -0.0907935

(0xF460E1) and Tgain = 42.3132 (0x54A05E) for a

Fahrenheit scale.

5.8 Voltage Reference

The CS5461A is specified for operation with a +2.5 V

reference between the VREFIN and AGND pins. To uti-

lize the on-chip 2.5 V reference, connect the VREFOUT

pin to the VREFIN pin of the device. The VREFIN pin

can be used to connect external filtering and/or refer-

ences.

5.9 System Initialization

Upon powering up, the digital circuitry is held in reset

until the analog voltage reaches 4.0 V. At that time, an

eight-XIN-clock-period delay is enabled to allow the os-

cillator to stabilize. The CS5461A will then initialize.

A hardware reset is initiated when the RESET pin is as-

serted with a minimum pulse width of 50 ns. The

RESET signal is asynchronous, with a Schmitt-trigger

input. Once the RESET pin is de-asserted, an

eight-XIN-clock-period delay is enabled.

A software reset is initiated by writing the command of

0x80. After a hardware or software reset, the internal

registers (some of which drive output pins) will be reset

to their default values. Status bit DRDY in the Status

and ready to receive commands.

5.10 Power-down States

The CS5461A has two power-down states, stand-by

and sleep. In the stand-by state all circuitry except the

voltage reference and crystal oscillator is turned off. To

return the device to the active state a power-up com-

mand is sent to the device.

In sleep state all circuitry except the instruction decoder

is turned off. When the power-up command is sent to

the device, a system initialization is performed (see

Section 5.9 System Initialization on page 20).

5.11 Oscillator Characteristics

The XIN and XOUT pins are the input and output of an

inverting amplifier configured as an on-chip oscillator,

as shown in Figure 8. The oscillator circuit is designed

to work with a quartz crystal. To reduce circuit cost, two

load capacitors C1 and C2 are integrated in the device,

from XIN to DGND, and XOUT to DGND. PCB trace

lengths should be minimized to reduce stray capaci-

tance. To drive the device from an external clock

source, XOUT should be left unconnected while XIN is

driven by the external circuitry. There is an amplifier be-

tween XIN and the digital section which provides

CMOS-level signals. This amplifier works with sinusoi-

off

∆

(

2.737649

– )

⋅

off

0.0951126

2.0

–

(

2.737649

– )

⋅

–

0.09566

–

---

17.7778

()

〈〉

---

gain

() TC

〈〉

off

17.7778

2.737649

–

⋅

()

[]

Oscillator

Circuit

DGND

XIN

XOUT

C1 =

22 pF

C2 =

Figure 8. Oscillator Connection

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