ICSP (In-Circuit Serial Programming), online serial programming, by keeping RB6 and RB7 pins, VDD is a program voltage, and increasing the MCLR (VPP) pin voltage from VIL to ViHH, device enters programming / Check mode. At this point, RB6 is the programming clock line, and RB7 is the programmed data cable. In this mode, both the RB6 and RB7 are the Schmitt trigger input, when RB7 drives data, it is a CMOS output drive.
After reset, in order to enter the program / check mode, the program counter (PC) points to the 00h address. A 6-bit command can then be sent to the device. According to this command, it is loaded or read, and 14-bit programming data will be provided to the device or from the device. However, in line serial programming mode, the watchdog timer circuit cannot generate a device reset.
Hardware circuit
Online serial programming circuit should pay attention to the following questions:
1) MCLR / VPP pin is isolated from other parts of the circuit
Usually the MCLR / VPP pin is connected to the RC circuit, and the pull-up resistance is connected to the VDD, and the capacitor is grounded. The VPP voltage must be isolated from the other part of the circuit, depending on the size of the capacitor, the RC circuit may affect the operation of the ICSP. Therefore, when the RC circuit is connected to MCLR / VPP, a Schottky type diode can be used to isolate the circuit. When programming for the PICmicro? MCL, the MCLR / VPP pin will be driven to approximately 13V, so the application circuit must be isolated from the programming voltage provided by the programmer.
2) Load of RB6 and RB7
The RB6 and RB7 pins are used for serial programming of the Picmicro® microcontroller. RB6 is a clock line, RB7 is a data cable. RB6 is driven by programmer, RB7 is a bidirectional pin, and is driven by a programmer when programming, and the check is driven by the PICMicro? These two pins must be isolated from the other parts of the circuit, thereby no impact on the signal when programming. The output impedance of the programmer must be considered when RB6 and RB7 are separated from other parts of the circuit. The isolation circuit must enable RB6 as an input to a Picmicro? And RB7 can be driven as a bidirectional pin (Picmicro® microcontroller and programmer can drive it).
3) Capacitor issues of VDD, MCLR / VPP, RB6 and RB7 pins
The total capacitance of the programming pin will affect the rising rate of the programmer output signal. In a typical circuit, a few hundred microscopic filter capacitors typically in VDD and the ground to suppress noise and supply voltage fluctuations. But such capacitors require programmers to have considerable driving capabilities to meet the requirements of VDD rise rates. Most programmers can only program the PICMicro? MCU, and cannot drive the entire application circuit. One solution is to add a driver circuit board between the programmer and the application circuit. The drive circuit board has a separate power supply, which meets the requirements of the VPP and VDD pin voltage rising rates and can be powered throughout the application circuit. RB6 and RB7 need to buffer depends on specific applications.
4) VDD minimum and maximum operating voltage
The Microchip programming specification specification device should be programmed under 5V voltage. If the application circuit can only work at 3V voltage, some special measures are required. For example, when programming, the PICmicro? Microcontroller is completely isolated from other application circuits. Another problem is that the device must be verified on the minimum and maximum operating voltage of the application circuit. For example, in a system that is powered by three 1.5V batteries, its operating voltage range is 2.7V to 4.5V. The programmer must be programmed on the 5V voltage and must be verified on the program memory at 2.7V and 4.5V to ensure proper programming. This ensures that the Picmicro® single chip microcomputer works normally over the entire operating voltage range.
5) Picmicro? Oscillator of the microcontroller
The PIC microcontroller is equipped with a power-derived timer on the code execution oscillator to wait 1024 oscillating cycles. The RC oscillator does not require the power-on delay time, so no upper electrical derived timer is used. The programmer must make the MCLR / VPP to enter the voltage required to enter the programming mode before the RC oscillator oscillates. If the RC oscillator oscillates 4 times or more, the program counter will increase to an uncertain value X. If the device enters the programming mode, the program counter is not zero, and the programmer will start bonding the code from the offset x. There are some ways to make up for the low rising rate problem of MCLR / VPP. The first method is to first connect the resistance of the RC oscillator, and then connect the R resistance after programming the device. Another approach is to pick up the Picmicro® OSC1 pin in the programming interface, so that oscillation is not generated during programming.
Consider the above situation, use the simplest way to implement the ICSP connection circuit of the PIC18F4550. In the circuit, the LEDs driven by the RD0 and the RD1 port are added, thereby verifying whether the burnning circuit can work normally.
For the ICSP interface and the PIKKIT® 3, the PIKKIT® 3 must be made corresponding to the pin of the ICSP.
PIC Microcontroller Online Serial Programming (ICSP), Read
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