Layout Notes - Intel PXA27 Series Design Manual

13.4

Layout Notes

The AC97_RESET_n signal is asserted when the PXA27x processor enters the sleep or deep sleep
modes resulting in the CODEC device being held in reset. This action results in CODEC devices
that have wake-up interrupt generating capability (such as touchscreen controllers) not being able
to utilize this functionality since the CODEC device is being held in reset. Also, for power
consumption considerations, holding a CODEC device in reset may not be its minimal power state.
One possible solution for avoiding having an external CODEC held in reset is to use the GPIO
functionality of the multiplexed GPIO113/AC97_RESET_n signal to control the CODEC reset
input signal while the PXA27x processor is in sleep mode. This is accomplished by performing
these steps sequentially before the PXA27x processor is in sleep mode:
1. Configure the AC97_RESET_n signal as GPIO[113] by setting the GPIO Alternate Function
Register 3 Upper field, GAFR3_U[AF113] bit to 0b00.
2. Configure GPIO113 as an output by setting the GPIO Pin-Direction Register 3,
GPDR3[PD113] bit.
3. Set the level of GPIO113 to output high by setting the GPIO Pin-Output Set Register 3,
GPSR3[PD113] bit.
4. Set the level of GPIO[113] to output high during sleep mode by setting the Power Manager
GPIO Sleep-State register, PGSR3[SS113] bit.
5. Enter sleep mode as outlined in Section 3 of the Intel
Manual.
Programming GPIO Alternate Function Register 3 Upper field, GAFR3_U[AF113], to 0b10 during
the wake up sequence restores the AC97_RESET_n functionality to the GPIO113/
AC97_RESET_n signal.
Because of the analog/digital nature of the CODEC, it is important to follow proper mixed-signal
layout procedure. Refer to the layout recommendations provided in the CODEC data sheet. Some
general recommendations are:
•
Use a separate power supply for the analog audio portion of the design.
•
Place a digital power/ground plane keep-out underneath the analog portion. Use a separate
analog ground plane. Create an island inside the keep-out. Connect the digital ground pins of
the CODEC to the digital ground. Keep the two ground planes on the same layer, with at least
0.125 inches (3.135 mm) separation between the ground planes.
Connect the two ground planes underneath the CODEC with a 0 Ω jumper. Add optional Do
•
Not Populate 0 Ω jumpers between analog and digital ground at the power supply. To reduce
excessive noise on the board, install the 0 Ω resistor.
•
Do not to route digital signals underneath the analog portion. Digital traces must go over the
digital ground plane, analog traces over the analog plane.
•
Buffer any digital signals to or from the CODEC that go off the board (for example, if the
CODEC is on a daughter card).
•
Fill the areas between analog traces with copper tied to the analog ground. Fill the regions
between digital traces with copper tied to the digital ground.
•
Locate the decoupling capacitors for the analog portion as close to the CODEC as possible.
®
Intel PXA27x Processor Family Design Guide
®
PXA27x Processor Family Developer's
AC '97
II:13-3