SLES025A datasheet(24/114 Pages) TI

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2–10

2.2.5

USB Suspend and Resume Modes

The TAS1020B can recognize a suspend state. Figure 2–2 shows the logical implementation of the suspend and

resume modes in the TAS1020B. The TAS1020B enters a suspend mode if a constant idle state (j state) is observed

on the USB bus for a period of 5 ms. USB compliance also requires that a device enter a suspend state, drawing only

suspend current from the bus, after no more than 10 ms of bus inactivity, The TAS1020B supports this requirement

by creating a suspend interrupt to the on-chip MCU after a suspend condition has been present for 5 ms. Upon

receiving this interrupt, the MCU firmware can then take the steps necessary to assure that the device enters a

suspend state within the next 5 ms.

There are two ways for the TAS1020B device to exit the suspend mode: 1) detection of USB resume signaling and

2) proactively performing a local remote wake-up event.

2.2.5.1 USB Suspend Mode

When a suspend condition is detected on the USB, the suspend/resume logic sets the function suspend request bit

(SUSR) in the USB status register, resulting in the generation of the function suspend request interrupt SUSR. To

enter the low-power suspend state and disable all TAS1020B device clocks, the MCU firmware, upon receiving the

SUSR interrupt, must set the idle mode bit (IDL), which is bit 0 in the MCU power control (PCON) register. Setting

the IDL bit results in the TAS1020B suspending all internal clocks, including the clocks to the MCU. The MCU thus

suspends instruction execution while in the idle mode.

The MCU must not set the IDL bit while in the SUSR interrupt service routine (ISR), or while in any other ISR. As

described in Section 2.2.5.3, it is intended that the receipt of an INT0 interrupt at the MCU result in exiting the suspend

state. But if the MCU has suspended instruction execution while in an ISR, subsequent INT0 activity is not recognized,

as the MCU is still servicing an interrupt. For this reason then, it is necessary that IDL not be set while processing

an ISR. (As described in Section 2.2.5.3, an external wake-up event will resume clocks within the TAS1020B. But

even if the clocks to the MCU resume, if the MCU does not recognize INT0, the IDL bit remains set and thus the MCU

core itself remains in the suspend state).

The SUSR bit is cleared while in the SUSR ISR by writing to the interrupt vector register VECINT. While servicing the

SUSR ISR, the VECINT output is 0x16 – the USB function suspend interrupt vector. As shown in Figure 2–2, the

occurrence of a write to VECINT, while the USB function suspend interrupt vector is being output, results in clearing

bit SUSR of the USB status register. (The data written to VECINT is of no consequence; the clearing action takes place

upon decoding the write transaction to VECINT).

2.2.5.2 USB Resume Mode

When the TAS1020B is in a suspend state, any non-idle signaling on the USB is detected by the suspend/resume

logic and device operation resumes. When the resume signal is detected, the TAS1020B clocks are enabled and the

function resume request bit (RESR) is set, resulting in the generation of the function resume request interrupt. The

function resume request interrupt to the MCU automatically clears the idle mode bit IDL in the PCON register, and

as a result the MCU exits the suspend state and becomes fully functional, with all internal clocks active. After the RETI

from the ISR, the next instruction to be executed is the one following the instruction that set the IDL bit. The RESR

bit is cleared while in the RESR ISR by writing to the interrupt vector register VECINT.

2.2.5.3 USB Remote Wake-Up Mode

The TAS1020B device has the capability to remotely wake up the USB by generating resume signaling upstream,

providing

the

host

has

granted

permission

generate

remote

wake-ups

via

SET_FEATURE

DEVICE_REMOTE_WAKEUP control transaction. If remote wakeup capability has been granted, the MCU firmware,

upon awakening from a suspend state, has to activate the remote wake-up request bit RWUP in the USB control

creates a pulse, which results in the TAS1020B generating resume signaling upstream by driving a k state (non-idle)

onto the USB bus. The USB specification requires that remote wake-up resume signaling not be generated until the

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