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Make QR Code 2d barcode in Software Figure 4.4 waveform.

Figure 4.4 waveform.
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Low-end PIC microcontroller fast verifying
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burning the program memory and when nished _MCLR is pulled low and cycled high again with the con guration fuse register programmed with its nal value. The reason for programming the con guration fuse register last is to make sure the code protect bit of the con guration register is not reset (enabled) during program memory programming. If code protection is enabled, then data read back will be scrambled during programming, which makes veri cation of the code impossible.
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A PIC17 microcontroller programmer connects to the chip as shown in Fig. 4.5. Note that PORTB and PORTC are used for transferring data 16 bits at a time and PORTA is used for the control bits that control the operation of the programmer. The _MCLR pin is pulled high to 13V as would be expected to put the PIC microcontroller into programming mode. While the programming of the PIC17Cxx is described as being in parallel, a special boot ROM routine executes within the PIC microcontroller and this accepts data from the I/O ports and programs the code into the PIC microcontroller. To help facilitate this, the TEST line, which is normally tied low, is pulled high during application execution to make sure that the programming functions can be accessed. The clock, which can be any value from 4 MHz to 10 MHz, is used to execute the boot ROM code for the programming operations to execute. To put the PIC microcontroller into programming mode, the TEST line is made active before _MCLR is pulled to Vpp and then 0x0E1 is driven on PORTB to command the boot code to enter the programmer routine (this sequence is shown in Fig. 4.6). To end programming mode, _MCLR must be pulled to ground 10 ms or more before power is taken away from the PIC microcontroller. TEST should be deasserted after _MCLR is pulled low.
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PIC17 parallel programmer connections.
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Figure 4.6 PIC17 parallel programming startup.
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When programming, the RA0 pin is pulsed high for at least 10 instruction cycles (10 s for the PIC microcontroller running at 4 MHz) to load in the instruction address followed by the PIC microcontroller latching out the data (so that it can be veri ed). After the data has been veri ed, RA0 is pulsed high for 100 s to program the data. If RA1 is low during the RA0 pulse, the PIC microcontroller program counter will be incremented. If it goes high during the pulse, the internal program counter will not be incremented and the instruction word contents can be read back in the next RA1 cycles without having to load in a new address. The latter operation is preferred and looks like the waveforms shown in Fig. 4.7.
PIC17 parallel programming waveform.
This waveform should be repeated until the data is loaded or up to 25 times. Once it is programmed in, then three times the number of programming cycles must be used to lock and overprogram the data in. This process is similar to that of the other EPROM parts. Writing to the speci ed addresses between 0x0FE00 and 0x0FE0F programs and veri es the con guration word. To program (make 0) one of the con guration bits, its register is written to. Reading back the con guration word uses the rst three RA1 cycles of Fig. 4.7 at either 0x0FE00 or 0x0FE08. Reading 0x0FE00 will return the low byte of the con guration word in PORTC (0x0FF will be in PORTB) and reading 0x0FE08 will return the high byte in PORTC. When writing PIC17 con guration fuse register bits, the addresses written to must be in ascending order. Programming the bit in nonregister ascending order can result in unpredictable programming of the con guration word as the processor mode changes to a code protected mode before the data is loaded in completely. This issue is important to watch out for in all PIC microcontroller programming; the con guration fuses must be programmed last, with any code protection programmed into the PIC microcontroller as the last possible programming operation. In some Microchip documentation, you will see comments that imply that the PIC17 has some ICSP or serial programming capability. This is not entirely correct as software called a bootloader (described later in the book) is used to save data passed to the PIC17 using the ability of the chip to write to its own EPROM program memory. This software can be used to provide a rudimentary in-circuit programming capability that can be exploited in your applications. The capability of a PIC17Cxx application to write to program memory is enabled when the _MCLR is driven by more than 13V and a tablwt instruction is executed. When tablwt is executed, the data loaded into the table latch (TABLATH and TABLATL) registers is programmed into the memory locations addressed by the table pointer registers (TBLPTRH and TBLPTRL). This instruction keeps executing until it is terminated by an interrupt request or _MCLR reset. To perform a word write, the following bootloader code execution sequence would be used:
1 2 3 4 5 6 7 8
Disable TMRO interrupts. Load TABPTRH and TABPTRL with the address. Load TABLATH or TABLATL with the data to be stored. Enable a 1,000 s TMRO delay interrupt (initialize TMRO and enable TMRO interrupt). Execute tablwt instruction with the missing half of data. Disable TMRO interrupts. Read back data; check for match. If no match, return error.
To enable internal programming, _MCLR has to be switched from 5V (Vdd) to 13V. The Microchip circuit that is recommended is shown in Fig. 4.8 and will drive the PIC17 s _MCLR pin at 5V until RA2 is pulled low. When RA2 is pulled low, the voltage driven in to _MCLR will become 13V (or Vpp). The programming current at 13V is a minimum of 30 mA.
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