Charging Neo Freerunner via USB port

[Michael Shiloh]

Hi everyone,

With input from the experts who designed the system, I've tried to 
document precisely how charging works on the Neo Freerunner.

I welcome your feedback:



The Neo Freerunner charges the battery when 5VDC is provided at the USB
port, whether from a computer USB port or from a dedicated USB charger.

The Neo Freerunner can charge most rapidly when it can pull 1 Amp from 
the power supply connected to the USB socket. However, not all chargers 
or computers can provide this much current.

When the Neo Freerunner detects that power has been provided at the USB 
port, it will attempt to draw only 100mA. This minimum is mandated by 
the USB standard. This amount of current is insufficient to both power 
the Neo Freerunner (or even just its backlight) and to charge the 
battery, and therefore the battery will not be charged. (The battery 
discharge rate, however, will be slightly lower, as the supplied 100mA 
will be used to augment the battery.)

(When a charger is connected to the USB port, the Neo Freerunner 
automatically powers up. Thus, if charging at 100mA is desired, the Neo 
Freerunner must be shut down after the startup process has completed.)

After detecting USB power, The Neo Freerunner will attempt to negotiate, 
via the USB protocol, a higher charge rate of 500mA. If the device 
powering the Neo is capable of doing so, the Neo Freerunner will charge 
at 500mA.

USB chargers do not implement the USB protocol, and thus can not respond
to requests for higher charge rates. Some manufacturers have worked
around this issue by installing resistors of different values between
different pairs of pins in in order to "identify" their own chargers of 
known capacity. This is not part of the USB standard and is completely 
up to each manufacturer.

The USB charger provided with the Neo Freerunner can source up to 1A. In
order to identify this special charger, there is a 47K ohm resistor 
between the ID pin and ground. If the Neo Freerunner detects this 
resistor, then the Neo Freerunner will charge at 1A.

In summary, the Neo can charge at 3 different rates: 100mA, 500mA, and 1A.

Notes:

1. USB negotiation and resistor detection is performed in software, and 
is thus under developer control. A developer might write an application 
to indicate that 500mA or 1 Amp are available, bypassing the USB 
negotiation and the 47K ohm resistor detection.

There is nothing  preventing the software from charging at a higher rate 
than then power provider can supply, although there is danger in doing so.

The danger in drawing more current than a charger or computer USB port
can provide is that components overheat and may become permanently
damaged, or even catch fire, although most USB host devices implement 
current limits that will depower the port on overcurrent.

2. The Neo Freerunner charger is a single assembly which includes
the USB cable. The cable is NOT a separate item and can not be removed
from the charger (without cutting).

3. Any third-party charger that does not contain the 47K resistor will 
cause the software to assume it can draw only 100mA, regardless of how 
much current the charger really can source.

4. In its hard-coded configuration, the PMU doesn't charge the battery
at all. The hard-coded configuration is used when power is applied to 
the PMU after a period of complete absence of power, including the 
backup battery.

When the system comes up, it reconfigures the PMU to enable charging. 
Most of the configurable items are also preserved by the PMU if it 
powers the system down, but the PMU itself still has power - either from 
USB, main battery, or the backup battery. (This is the PMU's STANDBY mode.)

5. All of this discussion is for setting the maximum current that the 
Neo Freerunner can safely draw from the USB socket. The battery charging 
current may actually be lower, if the charging logic determines that a 
lower charge rate is appropriate.