MS5540C

Miniature Barometer Module

09/2015

Page 13

SERIAL INTERFACE

The MS5540C 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 MS5540C on the rising edge of SCLK and each bit being sent by the

MS5540C 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 MS5540C with the falling edge of SCLK.

The SCLK-signal is generated by the

microprocessor’s system. The digital data provided by the MS5540C 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 Word1

(Figure 6c)



Calibration data read-out sequence for Word2

(Figure 6d)



Calibration data read-out sequence for Word3

(Figure 6c)



Calibration data read-out sequence for Word4

(Figure 6d)



RESET sequence

(Figure 6e)

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

MS5540C. 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.

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 READOUT 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 the module in any state recognizes its unique pattern. By consequence, it can

be used to restart if synchronization between the microcontroller and the MS5540C has been lost. This sequence

is 21-bit long. The DOUT signal might change during that sequence (see Fig. 6e). It is recommended to send the

RESET sequence before each CONVERSION sequence to avoid hanging up the protocol permanently in case of

electrical interference.

seq uen ce: START+P-measuremen t

Bit7

Conve rsion start for pressure measure ment and ADC-data-out "D1":

en d of co nv ersio n

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

co nv ersio n

(33 ms)

DB7

ADC-d ata ou tM SB

ADC-d ata ou tLSB

Bit8 Bit9

Start-bit

Sto p-b it

DB6 DB5 DB4 DB3 DB2 DB1

DB0 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

start o f co nv ersion

Setu p-b its

Fig. 6a: D1 ACQUISITION sequence