CS5535 datasheet(146/555 Pages) NSC

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Revision 0.8

4.15 GENERAL PURPOSE INPUT/OUTPUT

Proper use and understanding of the General Purpose

Input/Output (GPIO) subsystem is the key to applying the

CS5535 in a custom system design. By totalizing the

optional features of the CS5535 GPIOs, system functions

such as soft buttons, DDC monitoring, timers, system inter-

rupts, and others, may be implemented. The system

designer should pay careful attention to the suite of fea-

tures available through the GPIO subsystem and, because

the GPIOs are multiplexed with other on-chip functions,

must make careful trade-offs to obtain the features desired

in the system.

The register space for control of the GPIO subsystem con-

tains space for control of 32 GPIOs. Since only 28 GPIOs

are

realized,

the

control

bits

for

the

non-existent

GPIO[31:29], and GPIO[23] are marked “Reserved”.

GPIO[22:16] are multiplexed with the LPC bus; therefore, if

the system requires an LPC bus, GPIO[22:16] are not

available as GPIOs. Likewise, GPIO[15:14] are multiplexed

with the SMB (System Management Bus); if the system

requires the SMB, GPIO[15:14] will be dedicated to this

function and not available as GPIOs. Other GPIOs are mul-

tiplexed with individual functions as indicated in Table 2-8

"GPIO Options" on page 42.

Features

• Input Features:

— Each of the available GPIOs may be configured as

an input. A block of eight Input Conditioning Func-

tions, providing edge detection, event counting, and

input filtering, may be configured for use by any eight

of the 28 GPIOs, though all 28 may have edge detec-

tion. The optionally-conditioned input may then be

fed to steering logic that can connect it to an inter-

rupt, or power-management input event (PME).

• Output Features:

— Each of the available 28 GPIOs has a configurable

output cell. The output cell for each GPIO may be

independently configured to provide a variety of inter-

face options. The cell may be enabled or disabled,

configured as a totem-pole or open-drain type, have

internal pull-up or pull-down resistors applied, or be

inverted.

— As indicated in Table 2-8 "GPIO Options" on page 42,

the GPIOs have differing output driver types and

reset defaults. When choosing a GPIO for a given

function, choose one with a compatible output driver

type, and one that the use of, does not make another

desired function inaccessible. Careful study of this

table will assist the system designer in making proper

selections of the desired functionality of the suite of

GPIOs.

• Auxiliary Functions

— Most of the 28 GPIOs have additional hard-wired

internally-connected functions that may be selected

by choosing either the AUX_1 or AUX_2 outputs. Use

of these allows internal functions to be accessed at

the device pins. Table 2-8 "GPIO Options" on page

42 identifies these auxiliary functions, including

access to the UARTS and multi-function timers, as

well as certain power-management controls.

• Output Mapping:

— After passing through the optional input conditioning

circuits, any GPIO may be mapped (connected) to

one of eight PIC-level interrupts, or to one of eight

Power Management Event (PME) inputs. A given

GPIO may not be simultaneously mapped to both an

interrupt and a PME. The PIC subsystem interrupt

inputs may be configured to cause the generation of

an ASMI-type interrupt from any or all of the mapped

GPIO signals.

• Power Domains:

— The GPIO circuits are distributed into the Working

and Standby power domains. Those circuits in the

Standby power domain may be used for system

wakeup events, since they remain powered when the

Working power is removed. As indicated in Table 2-8

"GPIO Options" on page 42, GPIO[28:24] are located

in the Standby power domain; all others are in the

Working power domain. Event/Filter pairs 6 and 7

are located in the Standby domain; pairs [5:0] are in

the Working power domain.

• Auto-Sense:

— GPIO5 and GPIO6 have a feature called Auto-sense.

When reset is applied to the system, a weak internal

pull-up is applied to the pad. When reset is de-

asserted, the auto-sense value is used to establish

the pull-up/down state on the de-assertion edge. If

nothing pulls down the pad, then the weak pull-up

continues to be applied. If the pad is pulled down,

then pull-up is set to “no” and pull-down is set to

“yes”. The output driver does not actively drive the

pad, that is, it remains in TRI-STATE mode. If an

auto-sensed pull-down is desired, a diode between

the reset signal and the GPIO pin will pull it down

during the Auto-Sense operation but will have no

effect during normal operation.

• Recommended Functions:

— System designers at National Semiconductor have

created a list of recommended uses for selected

GPIOs, see Table 2-8 "GPIO Options" on page 42.

The desired functions were matched up with GPIOs

by selecting appropriate buffer types and multiplexing

options to create an optimal list of recommended

uses for the GPIOs. Designers may use these

recommended functions as a starting point and make

modifications to the list as needed to fit the particu-

lars of their system.

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