PCB design for Atmel AVR ATMega809 breakout board. Capabilities compatible
with Arduino Nano Every (ATMega4809) and all other ATMega{8/16/32/48}09
and x19 MCUs.
This provides more breakout pins from the 48-pin TQFP MCU than Nano Every or other Arduinos.
Power can be supplied in 3 ways:
- The USB port
VUSBis directly passed (unregulated) to the MCU - Power on
VINwill power a regulator that can operate at 5V0 or 3V3. - Directly provide power to the
VDDpin (unregulated)
Do not provide power to the regulator through VIN at the same time as
providing USB power through VUSB.
There are protection diodes so it should not immediately fail, but there is no regulator shut-off circuit that detects USB power, so there may be unanticipated consequences, especially if you plug in the USB power while the device is turned on already by VIN.
This is especially true if the regulator is configured for 3V3, as the MCU will immediately see its supply voltage increase while running.
VDD is also exposed on pin 15. You can use this to source power from the
linear regulator or to sink power back to the MCU. There is no diode or polyfuse
on this (or any) power bus of the device. Do not drive power into the device
through this pin at the same time as the regulator is powered.
The device is powered through an NCP1117LP adjustable 1A linear regulator.
This is configured for 5V0 operation by default. Closing the 3V3_SEL jumper will
reconfigure it for 3V3. A task best performed with the device unpowered.
- If powering the device for 5V0, you must supply 6.5V or greater on
VIN - If powering the device for 3V3, you must supply 5.0V or greater on
VIN VINabsolute maximum rating is 16V. For safe thermal operation, targetVDD + 3Vor less.
You can draw power from the regulator's output VDD on pin 15. The total
power available (including MCU power budget) is 1A.
The ATMega809 can drive up to 15mA per pin in 5V0 mode, and 7.5mA in 3V3. Absolute maximum rated drive per pin is +/-40mA. MCU absolute maximum VDD pin input is 200mA.
The main limit to current is based on the LDO's thermal regulation capability.
LDO power dissipation in this implementation has an absolute maximum 825 mW. LDO
power dissipated is given by (VIN - VDD - 0.35V)(IDD). Keeping VIN as close
to the 6.5V or 5.0V supply requirement gives more headroom for more current.
If VDD = 5V0 and VIN = 6V5, maximum current is 715 mA.
If VDD = 3V3 and VIN = 5V0, maximum current is 610 mA.
The MCU boots with CLK_MAIN driven by the OSC20M oscillator which is in
either 20MHz or 16MHz mode (check OSCFUSE bits to see how it is programmed),
with a CLKDIV of 6.
During boot, Arduino will disable the CLKDIV so your sketch is running at the full 20 or 16MHz.
If operating in low-voltage 3V3 mode, the maximum safe clock speed is 10MHz. So
a CLKDIV of at least 2 must be applied. If using an Arduino sketch on an
Arduino core compiled with default settings, the Arduino init() function will
fully unleash the clock divider for at least a short time before your setup()
method has time to pull it back to a divider of 2, which may cause boot
instability. Compiling the core with -DFCPU=10000000 will tell Arduino to
unthrottle the clock only as far as a divider of 2.
The breakout board offers I2C on the SDA/SCL pins. A dual-transistor buffer
isolates the pull-up voltages seen on these pins from those seen by the MCU.
That is, a 3V3 network can communicate with an MCU operating at 5V0 or vice
versa.
You must supply power to the VBUS pin in order to activate the buffer and
communicate over I2C. Otherwise the external I2C bus will be fully isolated from
the MCU I2C bus. If the MCU and the external circuit are operating at the same
voltage you can connect VBUS to VDD.
You must apply external pull-up resistors to SDA and SCL to use I2C.
Typical resistor value is 4K7. The bus capacitance added by the breakout
board is typically 36 pF (per PJA3438
datasheet)
[plus capacitance added by header pin, flying leads, etc].