barcode reader project in asp.net WDT Oscillator WDT Source Select Prescaler Reset in Software

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WDT Oscillator WDT Source Select Prescaler Reset
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Figure 9.15 The watchdog timer (WDT) reset signal can be passed through a prescaler to delay the onset of a timeout reset.
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It is recommend that the watchdog timer be reset by a single clrwdt instruction in the application after half the reset period has passed. The nominal error in the RC oscillator used for the watchdog timer function is 20 percent, which means that watchdog timer timeouts can take place after anywhere from 14 to 22 ms (when no prescaler is used). To be on the safe side, executing clrwdt every 9 ms in this situation will avoid any potentially invalid watchdog timer resets. The watchdog timer is enabled from within the con guration word and cannot be disabled within the application. This means that you have to be very careful to avoid enabling the watchdog timer unless you have provided support for it in the application code. Providing support for the watchdog timer means that the clrwdt instruction is executed repeatedly to prevent the watchdog timer from resetting the PIC microcontroller unexpectedly. In many PIC microcontrollers, the watchdog timer enable bit of the con guration word is positive active and enabled when the bit is set (i.e., unprogrammed). This can cause some problems for new application developers who forget to disable the watchdog timer explicitly.
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All PIC microcontrollers have the ability to protect the contents of memory either from being read back by programmers or from being altered inadvertently by an errant application when the application code has the capability of changing the contents of the program memory. Preventing data from being read back is a security procedure to thwart pirates from downloading and copying application code. In some PIC microcontrollers, the Flash program memory can be written to by the application; in some cases this is highly desirable, whereas in others it cannot be allowed. The programmer read-back code and self-write disable protection functions of the PIC microcontroller are controlled by con guration register bits and cannot be read back or altered by the application code. Code protection must be implemented only when the application code burned into the PIC microcontroller has been proven to be correct. With code protection enabled, you will not be able to examine a part during execution using ICD, and in the case of windowed EPROM parts, there is often an aluminum layer over the code-protection bits protecting them from ultraviolet (UV) light and rendering them impossible to reprogram. Only when you are absolutely sure that the code is correct should you enable the code-protection bits.
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Parallel memory devices can be connected to selected part numbers of the PIC17 and PIC18 microcontroller families to increase the program memory space available to the processor. In the PIC17, the interface provided is up to 64 kB of 16-data-bit words via a multiplexed address/data bus, and in the PIC18, up to 2 MB of data (either 8 or 16 bits wide) can be accessed. The multiplexed bus may seem somewhat dif cult to use, but it actually isn t. Memory devices can be added quite easily and quickly, and in the case of external Flash memory added to the PIC18 devices, the MPLAB ICD 2 can be used to program them with application code. Since the second edition of this book, the capabilities of the PIC18 devices have increased to the point where adding Flash memory to the application is a very viable alternative.
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BASIC OPERATING FEATURES
An unprogrammed PC17 s con guration fuses set the PIC microcontroller into microprocessor mode, which cannot access any internal program memory. This allows output devices to be placed into applications, with external program memory providing the application code. This feature presents a way to debug an application before it is burned into the PIC microcontroller. For both the PIC17 and PIC18 microcontrollers, external memory can be read from or written to using the TABLRD and TABLWT instructions. In both devices where there is both internal and external program memory, the internal program memory can be read using the TABLRD instruction in the microcontroller modes. When accessing data, these table instructions use the table pointer registers (TBLPTRU for bits 16 and above, TBLPTRH for bits 15 through 8, and TBLPTRL for the low 8 bits) to address the operation. During 16-bit table reads and writes, the table latch register (TABLATH for the high byte and TABLATL for the low byte) is used to buffer the 16 bits during the transfer. The PIC17 can only access data 16 bits at a time, whereas the PIC18 can access data either 8 or 16 bits at a time. Connecting external devices to a PIC17Cxx microcontroller is relatively simple, with two 74LS373 latches used for buffering the address before the I/O operation takes place (Fig. 9.16). The address bits can be decoded to provide access for multiple devices. A 74LS138 can be used to decode three lines into eight negative active outputs. When performing a read, the AD bus and ALE and _OE lines look like Fig. 9.17. These waveforms are actually quite traditional and match up with those of many microprocessors (such as the Intel 8086), but they should be reconciled with the waveforms for the TABLWT and TABLRD instructions presented earlier. Note that only one of the two data transfers will be visible on the PIC microcontroller s external bus. You also should note that two clock cycles are used for the data transfer. This means that the data access speed of the external device has to be less than twice the period of the PIC microcontroller s clock. For example, if the PIC microcontroller were running at 10 MHz,
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