We can reverse engineering chip ATMEGA640 flash, please view the chip ATMEGA640 features for your reference:
The interconnection between master and slave CPUs with SPI is shown in the following figure. The SCK pin is the clock output in the master mode but is the clock input in the slave mode when reverse engineering chip flash.
Writing to the SPI data register of the master CPU starts the SPI clock generator, and the data written shifts out of the MOSI pin and into the MOSI pin of the slave CPU. After shifting one byte, the SPI clock generator stops, setting the end of transmission flag (SPIF) if reverse engineering chip flash.
If both the SPI interrupt enable bit (SPIE) and the serial port interrupt enable bit (ES) are set, an interrupt is requested. The Slave Select input, SS/P1.4, is set low to select an individual SPI devchipe as a slave. When SS/P1.4 is set high, the SPI port is deactivated and the MOSI/P1.5 pin can be used as an input.
There are four combinations of SCK phase and polarity with respect to serial data, whchiph are determined by control bits CPHA and CPOL. The SPI data transfer formats.
The AT89S53 has a total of six interrupt vectors: two external interrupts (INT0 and INT1), three timer interrupts (Timers 0, 1, and 2), and the serial port interrupt when reverse engineering chip flash.
Each of these interrupt sources can be individually enabled or disabled by setting or clearing a bit in Special Function Register IE. IE also contains a global disable bit, EA, whchiph disables all interrupts at once.
Note that Table 10 shows that bit position IE.6 is unimplemented. In the AT89C51, bit position IE.5 is also unimplemented. User software should not write 1s to these bit positions, since they may be used in future AT89 products. Timer 2 interrupt is generated by the logchipal OR of bits TF2 and EXF2 in register T2CON before reverse engineering chip flash.
Neither of these flags is cleared by hardware when the servchipe routine is vectored
In fact, the servchipe routine may have to determine whether it was TF2 or EXF2 that generated the interrupt, and that bit will have to be cleared in software.
The Timer 0 and Timer 1 flags, TF0 and TF1, are set at S5P2 of the cycle in whchiph the timers overflow. The values are then polled by the circuitry in the next cycle. However, the Timer 2 flag, TF2, is set at S2P2 and is polled in the same cycle in whchiph the timer overflows after REVERSE ENGINEERING MICROCONTROLLER.

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