ST92195D datasheet(181/249 Pages) STMICROELECTRONICS

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ST92195C/D - SERIAL PERIPHERAL INTERFACE (SPI)

SERIAL PERIPHERAL INTERFACE (Cont’d)

7.9.8 IM-bus Interface

The IM-bus features a bidirectional data line and a

clock line; in addition, it requires an IDENT line to

distinguish an address byte from a data byte (Fig-

ure 101). Unlike the I2C-bus protocol, the IM-bus

protocol sends the least significant bit first; this re-

quires a software routine which reverses the bit or-

der before sending, and after receiving, a data

byte. Figure 100 shows the connections between

an IM-bus peripheral and an ST9 SPI. The SDO

and SDI pins are connected to the bidirectional

data pin of the peripheral device. The SDO alter-

nate function is configured as Open-Drain (exter-

nal 2.5K

Ω pull-up resistors are required).

With this type of configuration, data is sent to the

peripheral by writing the data byte to the SPIDR

user should write FFh to the SPIDR register, in or-

der to generate the shift clock pulses. As the SDO

line is set to the Open-Drain configuration, the in-

coming data bits that are set to “1” do not affect the

SDO/SDI line status (which defaults to a high level

due to the FFh value in the transmit register), while

incoming bits that are set to “0” pull the input line

low.

In software it is necessary to initialise the ST9 SPI

by setting both CPOL and CPHA to “1”. By using a

general purpose I/O as the IDENT line, and forcing

it to a logical “0” when writing to the SPIDR regis-

ter, an address is sent (or read). Then, by setting

this bit to “1” and writing to SPIDR, data is sent to

the peripheral. When all the address and data

pairs are sent, it is necessary to drive the IDENT

line low and high to create a short pulse. This will

generate the stop condition.

Figure 101. ST9 and IM-bus Peripheral

Figure 102. IM bus Timing

VDD

SCK

SDI

SDO

ST9 MCU

IM-BUS

CLOCK

DATA

IDENT

IM-BUS

SLAVE

DEVICE

PROTOCOL

PORTX

VR001427

2.5 K

CLOCK LINE

DATA LINE

MSB

VR000172

MSB

LSB

IDENT

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