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AT91SAM7S Microcontroller Circuit – Minimum Supporting Components

Created on: 25 October 2012

What is the minimum number of components required to build a circuit using AT91SAM7S ARM7 microcontrollers from Atmel?

This article and circuit diagram show the minimum number of supporting components required to build a circuit using the AT91SAM7S256 microcontroller.

Circuit Diagram

The circuit diagram of an AT91SAM7S256 microcontroller with the minimum supporting components is shown below:

AT91SAM7S circuit diagram with the AT91SAM7S256 ARM7 microcontroller from Atmel and minimum number of supporting components needed

AT91SAM7S Microcontroller Circuit Diagram Showing the AT91SAM7S256 Microcontroller and Minimum Supporting Components – click for a bigger image

Power Supply

Either an external 3.3V power supply to the circuit is required or a regulator circuit that steps an external voltage down to 3.3V.

AT91SAM7S microcontrollers generate their own 1.8V supply from the 3.3V input supply.


Some way of programming the microcontroller is needed once it is in the circuit or soldered on a board. The JTAG header is used for programming and debugging.

Erase and TST Pins

The logic state of the Erase pin can be changed by means of a jumper. This allows all Flash memory and fuse bits to be erased and reset to their default state. This pin could also be pulled permanently to GND.

The TST pin is only needed if using the on-chip SAM-BA bootloader. This pin is used to restore the bootloader to Flash memory so that software can be loaded to the microcontroller via the USB port. If the USB port is not being used, this pin can be permanently pulled to GND.


Each power supply pin is decoupled using a 100n X7R capacitor. The VDDIN pin has a 4.7uF X7R capacitor and a 100n capacitor connected to it as specified in the Atmel AT91SAM7S datasheet. The VDDOUT pin has a 2.2uF X7R capacitor and a 1n capacitor connected to it as specified in the datasheet.

PLL Filter and Crystal

The values of the RC filter components connected to the PLLRC pin need to be calculated depending on what clock speed the microcontroller will be operated at.

In the circuit diagram, an 18.432MHz crystal is used so that the master clock of the microcontroller can be run at 48MHz. This frequency is necessary if the USB device port of the microcontroller is to be used.