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Figure 9.16 The PIC17 external memory interface is quite simple, with two 373 latches used to buffer the lower address bits.
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CONFIGURATION FUSES
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Figure 9.17 The PIC17 memory read is quite straightforward.
the clock period would be 100 ns, and any external devices connected to the bus would have to have an access time of 200 ns or less. The PIC18 works very similarly to the PIC17, but there are a few differences that you should be aware of. The rst is that a JEDEC standard parallel-interface NOR Flash memory connected to the PIC microcontroller can be programmed using the MPLAB ICD 2. As part of this operation, the availability of external memory is selected during the MPLAB ICD 2 enable, and then during the programming operation, the device ID of the Flash chip is read and the appropriate programming algorithm is used. There are a few wrinkles to this operation, but these center around the PIC18 used and the features (such as BootRAM and internal Flash) that affect how the Flash is actually accessed. The ability to program Flash that is connected to the PIC microcontroller is a huge advantage when developing applications because it eliminates the need for a separate Flash programmer, as well as the need to convert the hex le to the data format required by the Flash chip and the programmer used with it. Second, the PIC18 can support either an 8- or 16-bit external memory device. The 16-bit read/write timing is similar to that of the PIC17, but the 8-bit timing is quite a bit different, as shown in Fig. 9.18. The 16-bit access is controlled by the BA0 pin of the PIC microcontroller, which is initially low to read the low byte of the 16 bits and then high to
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Figure 9.18 PIC18 16-bit memory read operation with BA0 switching to allow both bytes to be read in a single instruction cycle.
BASIC OPERATING FEATURES
initiate read of the high byte. The sequential read of the 2 bytes is done in the same instruction cycle, effectively halving the time available for the data to be output on the bus. This will be an issue when you are running the PIC microcontroller at high speeds; for example, at 48 MHz, the time available for the Flash to deliver the data will be less than 20 ns. To alleviate the potential for this problem, many devices allow for wait states to be inserted into the read process, allowing slower memory devices to be used.
PIC MICROCONTROLLER CONFIGURATION REGISTERS
When I discuss programming PIC microcontrollers, I recommend using only programmers that can pick up the value speci ed in the CONFIG statement and programming it into the PIC microcontroller automatically without any intervention from the user. This is important because it eliminates one step that is often missed or done incorrectly by the user. The __CONFIG MPASM directive (note that there are two underscores before CONFIG) is used to specify the con guration fuse options for the PIC microcontroller. Along with the __CONFIG directive, a number of constants are ANDed together to provide the con guration fuse values. A typical set of con guration fuse values (taken from the PIC16F877) is listed in Table 9.5 and can be found in the P[part number].inc. le that Microchip includes with MPLAB (where [part number] is the PIC microcontroller s part number). When working with other tools, there should be a similar include file with the same information. These values are meant to be ANDed together to get the actual bit pattern to be loaded into the con guration register. For example, for the watchdog timer to be disabled, bit 2 of the con guration word has to be reset. If you look through the label values in the preceding table, you ll see that this is the only parameter that has bit 2 reset. If the watchdog timer to be enabled, then the full 14 bits of the con guration word are set, which doesn t affect any other value ANDed with it. Because the different options controlled by the con guration register depend on bits being reset, I recommend that you select one of every option in the list in Table 9.5. This is done by matching up each parameter with the same character string inside both. For example, _CPD_OFF and _CPD_ON control to the same option, as does _LP_ OSC, _HS_OSC, _XT_OSC, and _RC _OSC. Table 9.6 lists the different option for the PIC16F877. After listing the different options, the __CONFIG statement for the application can be created in the application as follows:
__CONFIG _BODEN_ON & _CP_OFF & _WRTE_ENABLE_ON & _PWRTE_ON & _WDT_OFF & _XT_OSC & _DEBUG_OFF & _CPD_ON & _LVP_OFF
When this statement is evaluated, the constant values are replaced with the preceding constants and evaluated together into a constant to be loaded into the con guration fuses. For the labels selected above and replaced with the corresponding constants, the __CONFIG statement could be reduced to
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