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Voltage Clamping Diodes
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RS-232 Levels Input
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_MCLR/IO Pin 10 K
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Figure 15.6 Two clamping diodes and a 10-k resistor will allow the _MCLR pin with an internal reset to be used as an RS-232 input.
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INTERFACING TO EXTERNAL DEVICES
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In previous chapters I have given you a lot of information about the peripheral hardware built into the PIC microcontroller that will help to make applications easier. Coupled with the information contained in the appendices, you would have thought I have it all covered . . . In the following sections I want to go through some of the hints and tips I ve learned over the years for interfacing PIC microcontrollers with other devices. With this information, I have also included source code for the different interface hardware throughout this book. Much of this information and code will be used later in this book when I go through the experiments and projects. Some of the interfaces will seem very complex or dif cult to create, but I have tried to work through many of the dif culties and provide you with sample circuits that are simple and cheap to implement.
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MEMORY-MAPPED IO
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It should not be surprising that the PIC microcontroller can interface directly with TTL and CMOS digital logic devices. The PIC microcontroller s parallel I/O pins provide digital output levels that can be detected properly by both logic technologies and inputs that can detect logic-level output from these logic families. If you check the PIC microcontroller datasheets, you will see that the output characteristics are
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Vol ( output low voltage ) = 0.6 V (max) Voh ( output high voltage ) = VDD 0.7 V (min)
This speci cation is given to allow for different Vdd power inputs. For a Vdd of 5 V, you can expect a high output of 4.3 V or greater (normally I see 4.7 V when a PIC microcontroller pin is not under load). If the power voltage input (Vdd) is reduced to 2 V, low output still would be 0.6 V, and high output becomes 1.3 V (Vdd 0.7) or greater. The PIC microcontroller pins are speci ed to drive (source) up to 20 mA and sink (pull the output to ground). These current capabilities easily allow the PIC microcontroller to drive LEDs. The total current sourced or sunk by the PIC microcontroller should not exceed 150 mA (which is six I/O pins sinking the maximum current). The input threshold voltage, the point at which the input changes from an I to an O, and vice versa, also depends on the input power Vdd voltage level. The threshold is different for different devices. For a number of PIC microcontroller part numbers, this value is speci ed as being in the range
0.25 Vdd + 0.8V >= Vthreshold >= 0.48 Vdd
As a rule of thumb, you should use the higher value. For higher Vdds, this is approximately one-half Vdd. At lower Vdd voltages (2 V), the threshold becomes approximately two-thirds Vdd.
PIC MICROCONTROLLER APPLICATION DESIGN AND HARDWARE INTERFACING
PARALLEL-BUS DEVICES
While the PIC microcontroller is very well suited for stand-alone applications, there will be many applications that have to connect to external devices. While there are built-in PIC microcontroller interfaces for non-return to zero (NRZ) asynchronous I/O and twowire serial I/O, sometimes the best interface is a simulated parallel I/O bus. The parallel bus is useful for increasing the I/O capabilities of the PIC microcontroller using standard I/O chips. These devices can be accessed fairly easily using an 8-bit I/O port and a few extra control pins from the PIC microcontroller. I realize that PIC18 chips have the ability to drive a parallel bus, but it has only been recently that they have been able to access 8-bit devices (normally, they are designed for 16-bit data transfers), but the accesses take place as either two 8-bit transfers or may require using additional byte-selection pins to select between high and low bytes. If you are starting out with the PIC microcontroller, I would recommend that you tend to shy away from using these devices for this purpose because designing the circuitry for the transfers is quite involved and also requires a signi cant level of software architecting to make sure that the external devices do not interfere with any internal memory in the chip. It is actually much simpler to using the digital I/O pins to create a bus to access parallel-bus devices. When I create a parallel bus, I normally use PORTB for 8 data bits and other PORT pins for the _RD and _WR lines. To avoid the extra costs and complexity of decode circuitry, it is probably best to devote one I/O line to each device s chip select pin. Before writing from the PIC microcontroller to the device, TRISB is set to output mode, and the value to be written is output on PORTB. Next, the _CS and _WR lines are pulled low and remain active until the device s minimum access times are met. _RD is similar, with TRISB being put in input mode; the _CS and _RD pins are held active until the devices minimum read access time is met, at which point the data is strobed into the w register, and _CS and _RD are driven high. This is shown in the example circuit Fig. 15.7, which requires two parallel output bytes and one parallel input byte. This could be implemented with a 40-pin PIC microcontroller
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