PIC controller interface with a computer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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PIC controller interface with a computer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Microcontrollers

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When developing a device on a microcontroller (MC), the problem often arises of its connection with a computer for information exchange. In most cases, bidirectional mode is required at a relatively low exchange rate. In the best case, the MK can have a serial interface, but most often it has to be chosen from among cheap ones that are not equipped with such an interface. For example, Microchip's PIC16F84A MK, which is very popular lately, does not have such an interface. The article considers a variant of the software implementation of the serial interface both from the side of the MC and from the side of the computer.

To communicate with the device on the MK, you can use the parallel (LPT) or serial (COM) port of the computer. The first one is easier to work with - it can use a relatively larger number of input and output signals, the levels of which are compatible with TTL. The disadvantage of this port is that if under DOS or Linux simple I / O operations are enough to use it, then for correct operation under Windows, strict adherence to the data transfer protocol is required, which is not effective when working with MK. It is also possible to directly control individual lines of the LPT port, but this requires the installation of a special driver. The "disadvantage" of the LPT port can be considered that in most computers it is only one and, as a rule, is occupied by the printer.

The main advantages of the COM port are that the standard Windows programming interface (API) allows you to directly control some output lines and control the input ones, and also has the function of waiting for some event associated with the COM port. Its advantage is that the RS-232 standard, according to which COM ports are made, allows connecting and disconnecting cables during device operation (hot plug). In addition, almost always a computer has a free COM port. The disadvantage of the port is that the signal level is different from TTL, in which the low logic level corresponds to a voltage of -12, and the high logic level corresponds to +12 V.

The implementation of the standard RS-232 interface would require the MC to strictly observe the time intervals between the output signals. In a real situation, the microcontroller's quartz resonator may not correspond to the data transmission frequency, and the MC itself is usually busy with something more important than the formation of accurate time intervals. As a result, it turns out to be easier to programmatically implement a serial synchronous exchange option, when each data bit is confirmed by a synchronization pulse.

The schematic diagram of the proposed interface is shown in fig. one.