Datasheet

Analog Integrated Circuit Device Data

Freescale Semiconductor 92

MC34708

Functional Block Description

Figure 22. Stuck Key Process Flow Diagram

7.8.4.5 Power Supply Type Identification

The PSTI (Power Supply Type Identification) circuit is used in Active mode to identify the type of the connected power supply.

The PSTI circuit first detects whether the DP and DM pins are shorted. If the DP and DM pins are found to be shorted, the PSTI

circuit will continue to determine whether DP and DM pins are a forward short or reverse short. The detection result, together

with the ID detection result, is used to determine what powered accessory is connected.

The PSTI circuit is shown in Figure 23. Its operation is described as follows.

When the MC34708 detects the VBUS_DET bit is set, the PSTI identification flow starts.

1. Wait for a Detection Delay t

(programmable in the USB Time Delay register).

2. During t

, check to see whether ID_FLOAT = 0. If yes, then wait for the ID_DET_END to be set and check whether the

attached accessory is an A/V cable.

3. If the result is an A/V cable, set the A/V_CHG and ATTACH interrupt bits, as well as the A/V bit in USB Device Type/Status

4. Enable the PSTI (PSTI_EN set to '1') at t1. When PSTI_EN rises, the SW1 switch is turned on to drive the VDAT_SRC

data source voltage to DP line. In the meantime, the SW2 switch is turned on so the IDAT_SINK current source sinks a

current from the DM line. At t2, the PSTI starts to compare the DM line voltage with references VDAT_REF and VCR_REF.

If the DM line voltage stays above VDAT_REF, but below VCR_REF for 20

ms continuously before t4, which means the

DP and DM pins are shorted, the DP/DM_short signal is set to '1' at t3. Go to step 5. If the DP and DM are not shorted, the

VBUS detection completes at t4 and the VBUS_DET_END is set to '1'. The state machine will go to step 6 to determine

the type of accessory, based on the DM voltage.

5. The state machine checks if the ID pin is floating. If the ID pin is not floating at t3, the PSTI circuit turns off SW1 and SW2,

and the VBUS detection completes. The VBUS_DET_END is set to '1' and the state machine goes to step 6. If the ID pin

is floating at t3, the PSTI circuit turns off SW1 and SW2, and then turns on SW3 and SW4 to force VDAT_SRC to the DM

pin. If the DP pin is between the two thresholds VDAT_REF and VCR_REF for 20

ms continuously before t6, it means the

DP and DM pins are a reverse short.The DP/DM_reverse_short is set to '1' at t5, the SW3 and SW4 are turned off,

VBUS_DET_END is set to '1', and the state machine goes to step 6. If DP and DM are not a reverse short, the VBUS

detection completes at t6, SW3 and SW4 are turned off, the VBUS_DET_END is set to '1', and the state machine goes to

step 6.

6. The state machine decides on the attached accessory, based on the ID identification, and the VBUS identification results.