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AN S-7600A/PIC16F877 JOURNEY

Part 2: Revving It Up
by Fred Eady

Start ý Tera Term Pro ý Road Test ý The Hardware ý As a Web Server ý Branching Out ý Making Changes ý ReadyýSet? ý Go ý Sources and PDF

MAKING CHANGES

In Part 1, I mentioned that I would like to eventually place a Cermetek modem module at the S-7600A UART interface. The Cermetek modem of choice is the CH1786ET, which runs at 2400 bps maximum. The reasons for choosing 2400 bps are:

• The S-7600A/PIC16F877 Internet Engine wonýt be sending large chunks of data.
• 2400 bps takes little time to establish ISP connectivity compared to 56-kbps modems.
• The Cermetek modem runs at this speed.
• You can use almost any external modem for initial testing.

So, the S-7600A baud rate divisor can be calculated like this:

A simple substitution of known values gives you a data transfer rate divisor value of 0x005F.

Most of the time, the modem signals are rigged to simulate desired signal levels by crossing active pins with pins expecting activity on the DB shell connector. I chose to implement the S-7600Aýs modem control signals correctly because I may want to control data flow in the standard manner. The S-7600A serial port configuration register allows me to directly control DTR and RTS. Thereýs also a bit in the serial port config register that determines who has control of the S-7600A UART and serial port.

At this point, you arenýt ready to give control over to the S-7600A. The first order of business is to dial the ISP and do the PPP thing. So, 0x00 is sent to the S-7600A serial port configuration register and DTR and RTS are activated. The US Robotics modem has status LEDs and when the WriteSeiko(Serial_Port_Config,0x00 line is executed, LEDs on the modem for DTR and RTS illuminate. Nothing fancy is needed as far as modem configuration is concerned, so an AT&F is sufficient to set the modem straight.

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