DA5541B_005.doc

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00005541766 ECN865

SERIAL INTERFACE

The MS5541B communicates with microprocessors and other digital systems via a 3-wire synchronous serial

interface as shown in Fig. 1. The SCLK (Serial Clock) signal initiates the communication and synchronizes the

data transfer with each Bit being sampled by the MS5541B on the rising edge of SCLK and each Bit being sent

by the MS5541B on the rising edge of SCLK. The data should thus be sampled by the microcontroller on the

falling edge of SCLK and sent to the MS5541B with the falling edge of SCLK. The SCLK-signal is generated by

the microprocessor’s system. The digital data provided by the MS5541B on the DOUT pin is either the

conversion result or the software calibration data. In addition the signal DOUT (Data Out) is also used to indicate

the conversion status (conversion-ready signal, see below). The selection of the output data is done by sending

the corresponding instruction on the pin DIN (Data Input).

Following is a list of possible output data instructions:

• Conversion start for pressure measurement and ADC-data-out

“D1”

(Figure 6a)

• Conversion start for temperature measurement and ADC-data-out

“D2”

(Figure 6b)

• Calibration data read-out sequence for word 1

(Figure 6c)

• Calibration data read-out sequence for word 2

(Figure 6d)

• Calibration data read-out sequence for word 3

(Figure 6c)

• Calibration data read-out sequence for word 4

(Figure 6d)

• RESET sequence

(Figure 6e)

Every communication starts with an instruction sequence at pin DIN. Fig. 6 shows the timing diagrams for the

MS5541B. The device does not need a ‘Chip select’ signal. Instead there is a Start Sequence (3-Bit high) before

each Setup Sequence and Stop Sequence (3-Bit low) after each Setup Sequence. The Setup Sequence consists

in 4-Bit that select a reading of pressure, temperature or calibration data. In case of pressure- (D1) or

temperature- (D2) reading the module acknowledges the start of a conversion by a low to high transition at pin

DOUT during the last bit of the Stop Sequence.

Two additional clocks at SCLK are required after the acknowledge signal. Then SCLK is to be held low by the

microcontroller until a high to low transition on DOUT indicates the end of the conversion.

This signal can be used to create an interrupt in the microcontroller. The microcontroller may now read out the

16-Bit word by giving another 17 clocks on the SLCK pin. It is possible to interrupt the data read-out sequence

with a hold of the SCLK signal.

It is important to always read out the last conversion result before starting a new conversion.

The RESET-sequence is special as its unique pattern is recognized by the module in any state. By consequence

it can be used to restart if synchronization between the microcontroller and the MS5541B has been lost. This

sequence is 21-Bit long. The DOUT signal might change during that sequence (see Fig. 6e).

It is thus recommended to send the RESET-Sequence before each Conversion Sequence to avoid

hanging up the protocol permanently in case of electrical interference.

sequence: START+P-measurement

Bit7

Conversion start for pressure measurement and ADC-data-out "D1":

end of conversion

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

conversion

(33ms)

DB7

ADC-data out

MSB

ADC-data out

LSB

Bit8 Bit9

Start-bit

Stop-bit

DB6 DB5 DB4 DB3 DB2 DB1

DB0 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

start of conversion

Setup-bits

Fig. 6a: D1 acquisition sequence