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DEMYSTIFYING IN-CIRCUIT SERIAL PROGRAMMING

by Phil Anderson

Start ı Humble Beginnings ı Application Circuit Example ı The End Result ı Sources and PDF

APPLICATION CIRCUIT EXAMPLE

Letıs look at a real-life example. Keep in mind that some application circuits can get pretty complicated and it can be hard to isolate the chip from the circuit. In those cases, you may have to use an ICP interface that can supply sufficient current to the full application circuit. My example is simple, so you can focus on the question at hand.

The circuit consists of the ı16F873 microcontroller, some switches, a crystal circuit, supply leads, and the ICP SIP connector (see Figure 1). Assume that all of the other I/O lines are attached to circuitry. Note that the ICP SIP connector has five pins. These match with the five signals called for in the partıs datasheet for socket or ICP programming (ground, V_DD, V_PP, clock, and data).

The ICP SIP is simply a connector between the same pins on the in-circuit chip and the required pins of the programmer ZIF socket. The programmer ground attaches to V_SS of the chip, which is pin 19. V_DD attaches to the supply lead (pin 20), and V_PP attaches to pin 1. The programmerıs clock line attaches to RB6/PGC (pin 27), and the data line connects to RB7/PGD (pin 28).

If you look at the circuit carefully, youıll notice that the five signals required for programming are, or can be, isolated from the rest of the circuitry. In this way, the circuit does not load down the chip programmer (in my case, the PICSTART Plus).

V_PP drives MCLR high to 13 V, but diode D1, added to the circuitry, prevents V_PP from feeding back into the application circuit. When the programmer is removed, the diode doesnıt affect circuit operation.

To isolate the circuit from the programmerıs V_DD line, remove jumper J1. After programming, reinsert J1, restoring the application circuit. With regard to the programming clock and data lines, the application circuit shares B7 and B6. However, because the switches are normally open, their load is isolated during programming.

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