2.6 VCC

When programming the target microcontroller, the programmer outputs need to stay within the ranges specified in the DC Characteristics.

To easily adapt to any target voltage, the programmer can draw all power required from the target system. This is allowed as the In-System Programmer will draw very little power from the target system, typically no more than 20mA. The programmer shown in this application note operates in this mode.

As an alternative, the target system can have its power supplied from the programmer through the same connector used for the communication. This would allow the target to be programmed without applying power to the target externally.

Table 2-2. Recommendations when Designing Hardware Supporting In-System Programming
PinRecommendation
GNDConnect ground of the target to ground of the In-System Programmer
RESETAllow the In-System Programmer to Reset the target system
SCKWhen the target AVR microcontroller reset is active, this line should be controlled by the ISP Programmer.
Note: Edges on this line after Reset is pulled low, will be critical and cause the target AVR microcontroller to lose synchronization with the programmer.
When programming, oscillations on this pin should be tolerated by the surrounding system when the AVR Reset is active.
MOSIWhen the target AVR microcontroller Reset is active, this line should be controlled by the ISP Programmer. When programming, oscillations on this pin should be tolerated by the surrounding system when the AVR Reset is active.
MISOWhen the target AVR microcontroller Reset is active, this line should be allowed to become an output. When programming, oscillations on this pin should be tolerated by the surrounding system when the AVR Reset is active.
VCCAllow the In-System Programmer to draw power from the target system, to adapt to any allowed target voltage. The maximum current needed to power the programmer will vary depending on the programmer being used.