ARM hardware circuit design

The system reset module provides a start signal to ARM, which is the beginning of the operation of the entire system. The reset signal of ARM is RESET. If it is valid, the system reset will be generated internally. RESET suspends the program and puts ARM into the reset state. When the power is turned on and stable, RESET must remain at a low level for at least 4 MCLK cycles. This system uses a reset circuit designed with a capacitive resistor circuit, as shown in Figure (a). The key reset can also be designed as shown in Figure (b).

If the power chip has a reset pin, a low-level reset signal can be output for power-on reset, and the reset signal may not be used.

As shown in Figure (a), the working process of the reset circuit is as follows: when the system is powered on, the capacitor C1 is charged through the resistor R1. When the voltage across C1 does not reach the high-level threshold voltage, the Reset terminal output is a low level and the system is in a reset state; when the voltage across C1 reaches the high-level threshold voltage, the Reset terminal output is a high level and the system enters a normal working state.

When the user presses button S1, the charges at both ends of C1 are discharged, the Reset end outputs a low level, the system enters the reset state, and then repeats the above charging process, and the system enters the normal working state.

A two-stage NOT gate circuit composed of a 74HC32D chip is used for button debouncing and waveform shaping. The reset state time can be adjusted by adjusting the parameters of R1 and C1.

Figure ARM system reset circuit

2. Debugging of reset circuit

ARM is in reset state when powered on, and RS is low to reset the chip. In order to initialize the chip correctly, RS should be low for at least 5 CLKOUT cycles, which is about 125ns when the speed is 25ns. However, since the system's crystal oscillator often needs several hundred milliseconds to stabilize after power-on, the time that RS is low is mainly determined by the system's stabilization time, which is generally 100 to 200ms.

After the power supply part works normally, the ARM and its peripheral circuits such as the reset circuit should be debugged. Of course, simple output can also be performed. Two LEDs are used here, and these two LEDs are also the two control outputs of the terminal.

The process of testing the reset circuit is: disconnect the DE from the core board, that is, disconnect the JTAG debugging interface. Note that you should cut off the power supply on the board before disconnecting the JTAG interface cable. Then power on the core hardware board. At this time, the LED will flash. After the program is running, one LED will flash. When the reset button is pressed, the system is in the power-on initialization period. If the reset button is not released, all I/O ports will remain at a high level, and then both LEDs will light up; release the reset button and the program will restart. If the function is normal, the reset circuit is debugged.