AN1714 datasheet(31/46 Pages) STMICROELECTRONICS

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AN1714

Demonstration board for ST7538Q

31/46

If an emulator is linked to the board we recommend programming a 4 MHz clock in the

ST7538Q internal register.

For more accurate and complete information on the features, characteristics and issues

concerning ST microprocessors, please refer to the attached documentation or to the

reference documents or go to the site www.stmcu.com.

2.6.1

Modem / microcontroller interface

The interface signals between modem ST7538Q and the ST2C334 microcontroller can be

divided in three categories: the control signals, the communications signals and the auxiliary

signals.

The first group consists of the clock signal (MCLK/OSCIN) the reset signal (RSTO/RESET)

and the watchdog signal (WD/PD3).

The clock signal of the microcontroller is provided by the ST7538Q from the MCLK pin. The

default is 4Mhz but it is possible to increase this value (8 Mhz or 16 MHz) by programming

the ST7538Q control register.

The reset of the microcontroller is provided by the modem. The reset line is connected to the

manual reset (C22, R15 and SW1) and to the reset pin of the CN7 connector for the In-Situ

Programming mode procedures.

The watchdog signal has to be managed from the microcontroller (PD3 output port). If the

ST7538Q doesn't detect any activity on the WD pin, it generates a reset signal on the RSTO

pin. It is possible to disable this function through the modem control register.

The second group of signals consists of the links necessary for the modem/Micro Controller

Unit communication. These include the data signals RXD (from the modem to the MCU) and

TXD (from MCU to the modem), the transmitting/receiving status selection signal (RX/TX),

the internal ST7538Q register control access signal REG/DATA, and the recovery clock

signal CLRT.

The ISP (In Situ Programming mode) signals coming from the CN7 connector are also

linked to the communication wires and to the RESET. Remember to open the jumper J11

during the programming phase.

The simplest interfacing mode is the synchronous mode. In this case it is possible to use the

SPI interface of the MCU. The PC5/MOSI (Slave In Data) is connected to the RXD pin, the

PC4/MISO pin (Slave Out Data) is connected to the TXD pin and the PC6/SCKI (SPI serial

clock) pin is connected to the CLRT pin. The SPI Slave select (PC7/SS) is controlled by the

MCU itself through the PB0 I/O port.

The CLRT signal is connected to the PB1 I/O pin too.

It is also possible to implement an asynchronous interfacing mode, and for this reason the

pin RXD is also connected to the PC3/ICAP1_B pin (timer B input capture).

In this modality of communication the CLRT signal isn't considered and the recovered clock

has to be rebuilt internally by the MCU. If the ST7538Q control register has to be changed

from the default configuration, the first access has to be done at baud rate of 2400.

The idle state of the RXD output is the low state, so an asynchronous interface could be

necessary to invert externally this signal.

A diode (D13) and a pull down resistor R16 were inserted on the TXD connection line. With

these components it is possible to transmit a frame coming from an external device (for

example a BER tester). It is sufficient to configure the modem in a transmitting status and

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