SERIAL INTERFACE

MOTOROLA

MC33592 Technical Data

9

SERIAL INTERFACE

ROMEO2 and the microcontroller communicate through a Serial Peripheral Interface (SPI). It enables:

- the microcontroller to set and check ROMEO2 configuration,

- ROMEO2 to send the received data.

If the SPI is not used, a Power On Reset (POR) sets ROMEO2 to operate correctly in a default configuration.

The interface is operated by the 3 following input/output pins:

- Serial Clock SCLK,

- Master Output Slave Input MOSI,

- Master Input Slave Output MISO.

The master clock is used to synchronise data movement both in and out of the device through its MOSI and

MISO lines. The master and slave devices are capable of exchanging a byte of information during a sequence of

eight clock cycles. Since SCLK is generated by the master device, this line is an input on a slave device.

The MISO line is configured as an input in a master device and as an output in a slave device. The MOSI line is

configured as an output in a master device and as an input in a slave device. The MISO and MOSI lines transfer

serial data in one direction with the most significant bit sent first. Data are captured on falling edges of SCLK.

Data are shifted out on rising edge of SCLK. When no data are output, SCLK and MOSI force a low level. Using

Motorola acronyms, this means that the clock phase and polarity control bits of the microcontroller SPI have to be

CPOL= 0 and CPHA=1.

In configuration mode, as long as a low level is applied on RESETB (see state machine on figure 14 page 12),

the microcontroller is the master node providing clock information on SCLK input, control and configuration bits

on the MOSI line. If the default configuration is not the desired one, the microcontroller (MCU) can change it by

writing into the configuration registers. The configuration registers can also be read back to check their contents.

Configuration registers cannot be addressed separately, the whole configuration has to be sent as a 3x8

bitstream. The contents are written out as a 24-bit serial data stream. Transmissions which are not multiple of 24

bits may lead to unexpected configurations. The first bit transmitted on MOSI does not change the content of the

configuration registers. Note that a low level applied on RESETB does not affect the configuration register

content.

When RESETB is set to a high level, if Data Manager is enabled (DME=1), ROMEO2 becomes master and

sends received data on the MOSI line and the recovered clock on SCLK. It is then recommended that the MCU

SPI is set as slave. If the data received does not fit in an entire number of bytes, the data manager will fill the last

byte. If the data received constitute an whole number of bytes, the data manager may generate and send an

extra byte whose content is irrelevant. If DME=0, the SPI is disabled. Raw data is sent on the MOSI line.

When ROMEO2 SPI is changed from master (run mode) to slave (configuration mode) or from slave to

master, it is recommended that the MCU SPI is set as slave before the mode transition.

At power-on, the POR resets the internal registers. This defines the receiver default configuration (see gray

rows on tables 4, 7 & 8). In this configuration, the SPI is disabled and ROMEO2 sends raw data on the MOSI line.

This default configuration enables the circuit to operate as a standalone receiver without any external control.

After POR, RESETB forces a low level. Therefore an external pull-up resistor should be used in order to avoid

entering configuration mode.

Figure 12 : Writing into configuration registers

MOSI line

CR1

CR2

CR3

0

SCLK line

MCU (master)

ROMEO2

Freescale Semiconductor, Inc.

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