AVR Hardware Design Considerations [APPLICATION NOTE]
Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014
10
reset line. Despite the fact that the clock signal is deformed capacitive loads up to 1nF have been tested to work
with the Atmel STK600, AVRISP mkII and AVR Dragon™ during programming and the JTAGICE mkII and the
Atmel AVR ONE! during programming and debugging.
Pull-up resistors should be at least 10kΩ, or be removed from the RESET line if an Atmel programmer is used.
3.4 TPI Interface
The Tiny Programming Interface is featured on the Atmel tinyAVR
®
devices with the lowest pin count.
shows the standard TPI connector supplied with Atmel programmers. Only three pins on the device
are needed for use of this interface: RESET, TPICLK, and TPIDATA. The two latter pins are multiplexed with
regular I/O pins.
Figure 3-7.
Disabling RESET Pin Functionality
The RESET pin can be reconfigured as an I/O pin by programming the RSTDISBL fuse of the device. This
disables the reset functionality, and re12V to be applied to RESET for programming to work. Not all
programming tools are capable of generating this voltage.
4
Using Crystal and Ceramic Resonators
Most Atmel AVR MCUs can use different clock sources. The optional external clock sources are Clock, RC
oscillator, crystal or ceramic resonator. The use of crystals and ceramic resonators are in some designs causing
problems due to the fact that the use of these clock sources is not well understood. This section therefore treats
the topic of using crystals and ceramic resonators in relation to Atmel AVR MCUs. The description focus on
features and parameters relevant for designing applications where crystals or ceramic resonators are used
rather than trying to be a complete description of the theory related to the topic. For more information and theory
regarding crystals, refer to application note “
AVR4100: Selecting and testing 32kHz crystal oscillators for AVR
”.
4.1 Selecting the Clock Source in the AVR
The clock source used by the AVR is selected through the fuses for all parts except the Atmel AVR XMEGA
family where the clock source is selected by software. Most ISP and parallel programmers can program the fuses
that are related to selecting a clock source. The fuses are not erased when the AVR memory is erased and the
fuses therefore only require to be programmed if the fuse settings should be altered. Programming the fuses
each time the device is erased and reprogrammed is thus not necessary. The clock options that are relevant for
this document are:
•
“Ext. low-frequency crystal”
•
“Ext. crystal oscillator”
•
“Ext. ceramic resonator”
Several sub-settings relating to the start-up time of the AVR can be selected, but the three clock options
mentioned are the fundamental settings that should be focused on. The clock options available can vary between
AVR devices, - not all devices have the ability to run on various external oscillators. Check the datasheet for the
relevant device to determine the clock options.
