Power
 

Common to all boards

Initial Power on conditions
On power up all pins are tri-stated on the ARMexpress/ARMmite.

Restarting the program
If the user has programmed the ARMexpress/ARMmite, that program will be started when the power is applied, or restarted when RESET is asserted either low on the open-collector pin 22, or positive true on the ATN pin.

If the user program ends by getting to the last statement of the program or executing an END instruction, the ARMexpress will power down and await either input on the debug serial port, or a RESET.

Break operation or STOP
If the user code is running, it can be stopped by a RESET condition.  This will normally restart the user code, but there is a short window (500 msec) where the ARMexpress will wait to see if there is input on the serial debug port.  If the character received on the serial port is ESCAPE (27) or CTL-C (3)  then the user program is prevented from running and the ARMexpress is ready to be reprogrammed.  Or the user can restart the program by typing RUN or using the RUN button in BASICtools.

USB Power
The USB specification allows for up to 500 mA at 5V to be supplied to external devices.  In many cases this is limited to 100 mA by the manufacturer of the PC or hub.

ARMexpress and its eval PCB uses approximately 50 mA when running and 10 mA when idle.  So it can be powered from the USB port for programming, without the need for the alternate power supply.  The same is true for the ARMmite.

Once the programming is completed, the ARMexpress may be run without a connection to a PC.  In this case an alternate power supply connection has been provided.  This input goes to a regulator to supply 5V which is connected to pin 24 on the ARMexpress.  Onboard the ARMexpress this will be regulated to 3.3V and 1.8V for use by the ARM CPU.  The ARMmite takes this same unregulated input to generate either 5V or 3.3V on the rev2/rev3 versions respectively.

Smart Power

The USB evaluation board can be powered from either the USB, an external supply or BOTH.  Power from the USB is controlled such that it is turned on by the USB controller.  Power to the ARMexpress can also come from the external power supply and these are controlled to allow both USB and the power supply to be connected to the device at the same time.

The power connector is a 2.1mm, which is compatible with the Cui PP-002B part.

Battery backup

The ARMmite has a provision for adding a battery to keep its real time clock alive when power is removed.  The circuit is designed to use a Panasonic ML2020 rechargeable Li battery. 

Parallax STAMP compatibility

The Parallax STAMP products operate from a 5V supply.  This can come from an unregulated input on pin 24, or from a regulated 5V supply on pin 21.  The ARMexpress is backward compatible with both these connections, but for new designs it is recommended that power be supplied on pin 24.  The voltage required is 4.5V or greater on pin 24, or 5V on pin 21.   Also for C programming, pin 21 should not be connected to power.  The maximum voltage that may be applied to either pin 24 is 16V, but this is not a recommended continuous voltage, as it will cause extra heat to be generated by the ARMexpress onboard voltage regulators.  For this reason the recommended maximum is 9V.  When using an unregulated supply not supplied by Coridium, care should be exercised, as the current draw of the ARMexpress is low and the voltage will often be much higher than the rated voltage.  The user should ensure that this voltage does not exceed the limit of 16V.