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LOW-END PROGRAMMING
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The low-end PIC microcontroller requires at least 17 pins for programming, which are listed in Table 4.3. Figure 4.2 shows the block diagram for a programmer circuit that could be used in the low-end PIC microcontrollers. In this circuit, there are multiple single shots to ensure that the speci ed timing is achieved to program the PIC microcontroller for normal programming. A 100 s pulse is required, but for the con guration word, the timing is 10 ms, which is why I show the separate single shot. When a low-end PIC microcontroller is to be programmed, the _MCLR/Vpp line is pulled up to 13V, while TOCK1 is held high and OSC1 is pulled low. The PIC MCU s internal program counter (which is used for keeping track of the address) is initialized to 0x0100, which is the con guration fuse address. To program a memory location, the following procedure is used:
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1 The new word is driven onto RA0-RA3 and RB0-RB7. 2 The prog single shot sends a 100 s programming pulse to the PIC microcontroller.
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TABLE 4.3 LOW-END PIC MICROCONTROLLER PINS AND PROGRAMMING FUNCTION PINS PROGRAMMING FUNCTION
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RA3-RA0 RB0-RB7 T0CK1 OSC1 _MCLR/Vpp Vdd Vss
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D3-D0 of instruction word D11-D4 of instruction word Program/verify clock Program counter input Programming voltage PIC microcontroller power PIC microcontroller ground (Gnd)
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Figure 4.2 diagram.
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Low-end PIC microcontroller programmer circuit block
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3 The data word driver (driver enable) is turned off. 4 A programming pulse is driven, which reads back the word address to con rm the
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programming was correct. In Fig. 4.2, the read back latch is loaded on the falling edge of the on gate to get the data driven by the PIC MCU. 5 Steps 2 through 4 are repeated a maximum of 25 times or until the data stored in the latch is correct. 6 Steps 1 through 4 are repeated three times more than are required to get the correct data out from the PIC microcontroller. This overprogramming is used to ensure the data is programmed in reliably. 7 OSC1 is pulsed to increment to the next address. This operation also causes the PIC microcontroller to drive out the data at the current address before incrementing the program counter (which happens on the falling edge of OSC1). Looking at the circuit in Fig. 4.2, you are probably thinking that it is needlessly complex, and I would tend to agree with you if you were thinking of programming a low-end device using a dedicated intelligent programmer where timing pulse durations can be algorithmically produced. If you were going to use a programmer based on a PC s serial port, then the programmable single shot chips shown in Fig. 4.2 are de nitely required. In Fig. 4.3, the programming steps 1 to 4 listed above are shown along with the latch clock signal. When programming, there must always be two T0CK1 pulses, the rst being the programming pulse (10 ms or 100 s) and the readback. The program data word must be valid for one s before the T0CKI programming pulse is driven into the PIC microcontroller, and data out is available 250 ms after the falling edge of T0CKI. Note that using the circuit shown in Fig. 4.2 will result in data being driven into the readback latch because T0CKI is used for the programming pulse.
PROGRAMMING PIC MICROCONTROLLERS
Figure 4.3 waveform.
Low-end PIC microcontroller programming
There are four things to note about low-end PIC microcontroller programming. The rst is that when _MCLR is active at 13V, the program counter is initially set to the con guration fuse register this is different from the mid-range devices. The con guration register also requires a considerably longer pulse to program than the standard addresses. Secondly, just pulsing the OSC1 pin can be used to implement a fast verify, as shown in Fig. 4.4. As noted above, each time OSC1 is pulsed, data at the current address will be output and then increment the PIC MCU s program counter. Figure 4.4 shows the fast verify right from the start with the con guration fuse output rst to be veri ed before the contents of the program memory. Past the end of the low-end PIC microcontroller s program memory are 4 bytes of EPROM words that can be used for serial number or application code version information. These four words cannot be accessed by the PIC microcontroller during application execution and are known as the ID location or IDLOCS. The last point to make is that the con guration fuse register should always be programmed last. This means that the con guration information is skipped over when
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