AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
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Figure 2-2.
Switch Connection for Reset Pin
3
Connecting ISP Lines
Atmel AVR microcontrollers feature one or more interfaces for In-System Programming (ISP). These are used for
programming the Flash, EEPROM, Lock-bits, and most Fuse-bits in all AVRs (except the Atmel ATtiny11 and
ATtiny28). This feature makes it possible to program the AVR in the last stage of production of a target
application board, reprogram if SW bugs are identified late in the process, or even update the AVR in the field if
required. Some of the ISP interfaces may also be used for on-chip debugging. It is therefore highly
recommended to always design the target application board so that the ISP connectors can be accessed in some
way — just in case.
The different programming interfaces are introduced in the following subsections. Refer to the device datasheet
and Atmel AVR Tools help for further information on which interfaces are supported by the device, and how to
connect the programming tool.
3.1 SPI Programming Interface
On devices that use a Serial Peripheral Interface (SPI) for ISP, these lines are usually located on the same pins
as regular SPI, or else on pins that can be used for other purposes. Consult the device data sheet to find out
which pins are used for ISP.
Two standard SPI connectors are provided by the Atmel ISP programmers; a 6-pin and a 10-pin connector.
These are seen in
. In addition to the data lines (MOSI and MISO) and the bus clock (SCK), target
voltage VTG, GND, and RESET (RST) are provided through these connectors.
Figure 3-1.
Standard ISP Connectors on STK500, AVR ISP, and STK200/STK300
Some ISP programmers are powered by the target power supply. In this way they easily adapt to the correct
voltage level of the target board. Other ISP programmers, like Atmel STK500, can alternatively power the target
board via the VTG line. In that case it is important that the power supply on the target is not switched on. Read the
user guide for your ISP programmer for details on its capabilities and physical interface.
