STC MCU is an enhanced 51 MCU, fully compatible with MCS-51, also adds new features, such as adding two levels of interrupt priority, one more external interrupt, built-in EEPROM, hardware watchdog, with power-down mode, 512B memory and so on. Also support ISP download, without a programmer, as long as a MAX232 and some inexpensive components can write programs that can be erased 100,000 times. Therefore it is a very good microcontroller.
Stc microcontroller advantages:
1, strong anti-interference ability;
2, security performance is tough, it is difficult to be cracked;
3, SCM clock protection against external electromagnetic radiation;
Stc SCM disadvantages:
1, high power consumption, 5V power supply;
2, 8 bits, slow operation;
3, no hardware multiplier, multiplication and division operations are four-cycle instructions;
4. There are few integrated resources in the microcontroller.
Why STC Microcontrollers Want Cold StartupThe stc program is divided into a boot area and a program area. The boot area code is mainly responsible for ISP online burning. The cold start sequence of stc is to boot from the boot area and then to the program area. The hot start is the normal 51 startup mode, which can only be restarted from the head of the program area. So you must start cold. In addition, the STC provides a special register address, and jumps directly to the boot area by setting a function that he can implement by a software hard reset.
What is the cold start and reset of the STC single-chip microcomputer? The STC12 series is used as an example.
Cold start, power down the SCM, power up after power is discharged, and it is a cold start.
Reset, the microcontroller does not power down, reset it using the reset pin (there are other types of reset: watchdog, IAP_COUNTER control soft reset). These are hot start.
STC microcontroller reset modeTake the STC12C5A60S2 series of microcontrollers as an example:
1. The external RST pin resets.
2, external low pressure detection reset.
3, software reset
4, power-off reset / power-on reset
5, watchdog reset
The difference is that the reset start is forced to restart (the premise is that your computer must be already on), and the cold start is that you shut down and start again, this is in accordance with the working principle of the computer (hard disk).
There is also a reset start that forces a reboot regardless of whether your hard disk is running or not. This will damage your hard disk. The cold start will wait for the computer (hard disk) to save the data and start it again, without damaging the hard disk.
Cold start download and reset on stc51Stc single-chip microcomputer inside a solidified program, the role of this program is to detect whether the serial port to download the program, do not need to implement the user program in the microcontroller. This program runs every time it starts, which is why it is a cold start every time it is downloaded. After reset, the MCU starts executing at address 0000H. The address 0000H will point to the main program entry point, that is, the main function, that is, the user program downloaded on the chip and will not execute the previously solidified serial detection routine. This is why the microcontroller must be cold-started each time it is downloaded, but not reset. Note that what the macro in front of the program defines does not take up system time, so the main program is the main function.
There is a way to give users who need to power up immediately. The STC MCU can be set to start the program download only when P1.0/P1.1 is low when powering up. Otherwise, the user program can be executed directly. This approach is fast, without waiting for the period of time before the serial port is detected. The cold start of the stc microcontroller is necessary and the reset circuit is not necessary, either cold or manual. Just want to download the program must be cold start, and there is no reset circuit does not matter. But the minimum system board must have a reset circuit, whether it is a power-on reset, or power-on plus manual reset, although in some simple programs can not see the difference, but because the reset operation will affect some special registers, so there is no reset If you execute the function again, you will get an error. After power off, the microcontroller will continue to execute from the main function. It may be that the address of the microcontroller automatically returns to 0000H after power off, but the values ​​in those special registers will not change.
For a power-on reset, it looks like the circuit will not have a current flowing through it. However, at the instant of power-up, there is a process of charging the capacitor. During this process, a positive pulse will appear on the rst port. As long as the pulse is maintained for more than 1ms, the microcontroller can be reset.
The reset operation of the microcontroller causes the microcontroller to enter the initialization state, which includes making the program counter PC = 0000H, which indicates that the program starts execution from the 0000H address unit. After the microcontroller is cold-started, the on-chip RAM is a random value. The reset operation in operation does not change the contents in the on-chip RAM area. The status after resetting the 21 special function registers is the determined value, see the following table.
It is worth pointing out that, remembering the main state of some special function registers after reset, it is very necessary to reduce the initialization part of the application program to understand the initial state of the microcontroller. Explanation: The symbol * in the table is a random state; A=00H, indicating that the accumulator has been cleared;
PSW = 00H, indicating that register 0 is a working register group;
SP=07H, indicating that the stack pointer points to the on-chip RAM 07H byte unit. According to the stack operation, the first pressed input is written into the 08H unit.
Po-P3=FFH, indicating that 1 has been written to each port line. In this case, each port can be used for both input and output. IP=×××00000B, indicating that each interrupt source is at low priority; IE=0× ×00000B, indicating that each interrupt is turned off;
A system reset is the first step in the execution of any microcomputer system, allowing the entire control chip to return to the default hardware state. The reset of the 51 MCU is controlled by the RESET pin. After this pin is connected to the high level for more than 24 oscillator cycles, the 51 MCU enters the internal reset state of the chip and waits until the RESET pin. After turning to low level, it is checked whether the EA pin is high level or low level. If it is high level, the internal program code of the chip is executed. If the level is low, the external program will be executed.
In the system reset, the 51 single-chip microcomputer sets some important registers in its internal memory to a specific value, and the data in the internal RAM remains unchanged.
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