With International Telecommunications Union (ITU ) agreeing a common approach to universal phone chargers, the Micro-USB connector is well on the way to becoming the universal wired interface for phones and other portable gadgets.
Unlike the standard USB connector, which has four connections, the Micro-USB plug and receptacle have five: ground, +ve supply, data+, data- and identification (ID).
USB was originally a way to connect PCs to peripherals, and OTG was invented for situations when two devices, neither of which is a PC, are to be connected together - a camera and printer for example.
There are two slightly different Micro-USB connectors and the OTG lead has a Micro-A plug in one end with the ID pin connected to ground (<10Ω), and a Micro-B plug on the other end with its ID pin open circuit (>100kΩ).
Peripheral-only devices have a B receptacle which will physically block the host (A) end of the cable.
Using the ID pin with a resistor to ground rather than a short or open circuit allows phone makers to turn their Micro-USB receptacle into a port for analogue and non-USB data functions.
For example, the Japanese Mobile Computing Promotion Consortium (JMPC) suggests 280kΩ indicates that a stereo headset with a microphone is connected, and that this be made of a 47kΩ resistor in series with a 240kΩ resistor if a button to answer and terminated calls is included in the headset - with the button shorting out the 240kΩ resistor - see table.
There is a compatibility issue here.
OTG-enabled devices - not supported by the JMPC specification - can put power out of their USB +ve terminal, so JMPC suggests microphones are designed to be survive this connection.
Chips designed to decode the ID resistors are not that common.
Freescale's MC34825, introduced two years ago, is one: an in-phone Micro-USB port handler with power and data routing switches, as well as an ADC that can differentiate 32 different resistor values.
12 values up to 24kΩ in the chip, for example, are allocated to up to 12 push buttons built into accessories, and the values run up to 500kΩ.
Although some of the ADC output values are decoded on-chip to help with charge current routing, the majority of them are passed to the phone's microcontroller which then sets the chip's analogue switches (see diagram) to route the data+ and data- connections to the USB transceiver, a UART, or the audio headphone driver - with or without microphone.
Far more common, are power chips that have been designed to work with the original Micro-USB charging proposal, and those that diverged from it.
For example, Texas Instruments' TPS2540 supports: USB2.0 battery charging specification BC1.2, Chinese telecommunications industry standard YD/T 1591-2009, and 'divider mode' - where a two tap potentiometer between the positive rail and ground sets voltages on the data+ and data- pins to indicate a charger is present.
There is also a scheme which shorts data+ to data- to show a charger is present.
The new ITU USB charging specification, not to be confused with the one it published last year, is touted as a likely global standard.
Which signalling system, or systems, it uses will be revealed when it is published - likely to be before the end of June.
What is known is that it has been extended to cover the charging of MP3 players, games, video players and cameras by charge current above the 500mA specified in the USB standards, and that there will be two phases to the introduction.
Until some time in 2014, complaint chargers will have a maximum stand-by power of 150mW (4 star) and deliver a maximum output current between 500mA and 1.5A - and be allowed to have permanently attached cable.
From then onwards, standby power must be less than or equal to 30mW (5 star), maximum output current will have to be at least 750mA, and the cord will have to be detachable with a full-sized USB connector on the other end to allow it to double as a data cable for device-computer interaction.
| Mode \ Pin name | V_BUS | d- | d+ | ID | GND |
|---|---|---|---|---|---|
| Stereo earphone and mic | Mic or open | L ear | R ear | 47k or (47+240k) | GND |
| Mono earphone and mic | Mic or open | L+R ear | NC | 47k or (47+750k) | GND |
| Stereo earphone and charge | V_BUS | L ear | R ear | 47k or (47+240k) | GND |
| Mono earphone and charge | V_BUS | L+R ear | NC | 47k or (47+750k) | GND |
| Mono earphone with mic and charge | V_BUS | L+R ear | Mic | 47k or (510k+47k) | GND |
| Reserved | . | . | . | 390k | . |
| Reserved for device maintenance | . | . | . | 180k | . |
The Japanese Mobile Computing Promotion Consortium proposes these identification resistor values in its TR-014 document. It allows for dual-mode ports that can recognise the presence of a charger and re-assign the microphone pin.
47k = 'Call' button active
Analogue switches in Freescale's MC3425 IC connect pins on a phone's Micro-USB receptacle to 480Mbit/s USB, UART data or headset and microphone depending on the resistance presented between the accessory's ID and ground pins. The mosfet switches charging current
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