2.2 RESET
The target AVR microcontroller will enter the Serial Programming mode only when its reset line is active (low). When erasing the chip, the reset line has to be toggled to end the erase cycle. To simplify this operation, it is recommended that the target reset can be controlled by the In-System Programmer.
Immediately after Reset has gone active, the In-System Programmer will start to communicate on the three dedicated SPI wires; SCK, MISO, and MOSI. To avoid driver contention, a series resistor should be placed on each of the three dedicated lines if there is a possibility that external circuitry could be driving these lines. The connection is shown in the figure below ("Connecting ISP Programming Cable to Target SPI Bus"). The value of the resistors should be chosen depending on the circuitry connected to the SPI bus.
Pin | Name | Comment |
---|---|---|
SCK | Serial Clock | Programming clock, generated by the In-System Programmer (Master) |
MOSI | Master Out – Slave In | Communication line from the In-System Programmer (Master) to the target AVR being programmed (Slave) |
MISO | Master In – Slave Out | Communication line from the target AVR (Slave) to the In-System Programmer (Master) |
GND | Common Ground | The two systems must share the same common ground |
RESET | Target AVR MCU Reset | To enable In-System Programming, the target AVR Reset must be kept active. To simplify this, the In-System Programmer should control the target AVR Reset. |
VCC | Target Power | To allow simple programming of targets operating at any voltage, the In-System Programmer can draw power from the target. Alternatively, the target can have power supplied through the In-System Programming connector for the duration of the programming cycle. |
To avoid problems, the In-System Programmer should be able to keep the entire Target System Reset for the duration of the programming cycle. The target system should never attempt to drive the three SPI lines while Reset is active.