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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

THE HARDWARE

The heart of the S-7600A/PIC16F877 Internet Engine is the S-7600A. The S-7600A contains all of the necessary hardware and firmware to implement Internet protocols such as TCP/IP, UDP, and PPP. In addition to 10 KB of on-chip RAM to support the protocol stack, the S-7600A has its own UART. Support circuitry for the S-7600A consists of a 74HC4040 that divides the incoming 7.3728-MHz processor clock by 32 and a Sipex SP3243E RS-232 converter.

Data and command information is supplied to the S-7600A by a Microchip PIC16F877 microcontroller. The S-7600A can take information from the PIC serially or with an 8-bit parallel configuration. Because the PIC16F877 has a wealth of I/O, I chose the parallel attachment method. The PIC is clocked at 7.3728 MHz and provides clocking for the S-7600A through the 74HC4040, which presents 230 KHz to the S-7600Aýs clock input. A Sipex SP3243E RS-232 converter IC that allows the PICýs UART to interface with Tera Term Pro running on a PC also supports the PIC16F877.

Boot loader code is initially loaded onto the PIC16F877 using a standard PIC programmer and a header/connector ribbon cable assembly like the one shown in Photo 5. My PIC programmer uses wide Aries ZIF sockets so I can make my programming jig using a standard 0.600 header. The connector you end up with depends on your PIC programmer setup.

Photo 5ýThese two signal lines, power and programming voltage lines, are standard fare for programming PICs.

The important thing is to have the necessary signals and voltages between the header and S-7600A/PIC16F877 Internet Engine for programming the PIC16F877. The business end of the in-circuit programming connector on the S-7600A/PIC16F877 Internet Engine is shown in Photo 6.

Photo 6ýThe extra pins are connected to uncommitted I/O lines on the PIC16F877.

If I used the space wisely, the PIC16F877 has enough on-chip data EEPROM to accommodate the parameters needed to make a connection to the Internet. However, there wouldnýt be enough space left to put a decent HTML image in this memory area. So, a Microchip 24LC256 EEPROM is employed to hold all of the text necessary to connect to the Internet and serve an HTML-based page.

Just to make things interesting, I decided to throw in a Dallas DS1629 time and temperature IC. All the DS1629 needs to operate is a standard 32.768-KHz crystal and some room on the I²C bus. The DS1629 comes hard addressed and uses the extra pins that would normally be address lines for crystal and alarm connections. The 24LC256 is addressed as 0x00 and the DS1629 is addressed at 0x07. The DS1629 clock is volatile and must be backed up with a 3-V lithium cell if you expect to lose power to your S-7600A/PIC16F877 Internet Engine after it is deployed. And, speaking of power, a National LM2937 3.3-V regulator and a standard 9-VDC power brick powers the whole thing. You can see how it all fits together by perusing the schematic shown in Figure 1. The real McCoy is shown in Photo 7.

Photo 7ýThis is the first pass of the circuit board. Iýll officially make room for the lithium cell on pass two.

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