LM8322 datasheet(25/42 Pages) NSC

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16.0 Host Interface

The two-wire ACCESS.bus interface is used to communicate

with a host. The ACCESS.bus interface is fully compliant with

the I2C bus standard. The LM8322 operates as a bus slave

at 400 kHz (Fast mode).

All communication with the LM8322 over the ACCESS.bus

interface is initiated by the host, usually in response to an in-

terrupt request (IRQ low) asserted by the LM8322. The

LM8322 may request service from the host by asserting the

IRQ interrupt output.

16.1 START AND STOP CONDITIONS

Every transfer is preceded by a Start condition or a Repeated

Start condition. The latter occurs when a command follows

immediately upon another command without an intervening

Stop condition. A Stop condition indicates the end of trans-

mission. Every byte is acknowledged by the receiver.

30013615

FIGURE 15. Start and Stop Conditions

16.2 CONTINUOUS COMMAND STRINGS

A host device may send a continuous string of commands

using the Repeated Start condition, which would block an-

other ACCESS.bus device from gaining control of the bus.

After Power-On the host device must send multiple com-

mands to initialize the LM8322 device. A minimal command

string will include the commands shown in Table 10.

TABLE 10. Minimal Command String

Command

Description

READ_ID

Read vendor ID and software

version

READ_INT

Check if NOINT bit is set in

interrupt register

WRITE_CFG

Configure the LM8322

SET_KEY_SIZE

Set the size of the keypad

WRITE_CLK

Set the clock mode for the PWM

unit

WRITE_PORT_SEL

Set port direction for GPIO pins

WRITE_PORT_STATE Set port states of GPIO pins

A more comprehensive command string may include the ad-

ditional commands shown in Table 11.

TABLE 11. Additional Commands

Command

Description

SET_DEBOUNCE

Set debounce time

SET_ACTIVE

Set active time

READ_CLK

Verify PWM clock settings

READ_CFG

Verify configuration setting

READ_PORT_STATE

Read all port states (physical levels

on pins)

Note: Very long continuous command strings exceeding 30

milliseconds could overrun the ability of the LM8322 to pro-

cess commands if the time from the last clock cycle of a

command until the next Start condition or Repeated Start

condition is always shorter than 60 µs. A very long command

chain could prevent the LM8322 from performing any watch-

dog service and consequently could trigger a physical

RESET to the device.

To avoid overrunning the LM8322, the host should not send

a Start condition or a Repeated Start condition less than 100

µs after the last Stop condition or the last clock of a preceding

command.

16.3 DEVICE ADDRESS

The device address is controlled by states sampled on the

CONFIG_1 and CONFIG_2 pins, as shown in Table 12. In the

first byte of a bus transaction, a 7-bit address plus a direction

bit are broadcast by the bus master to all bus slaves.

TABLE 12. Device Address Selection

CONFIG_1

CONFIG_2

Device Address

1000 010X

1000 011X

1000 100X

1000 101X

If the CONFIG_1 and CONFIG_2 pins are left open, on-chip

pullups will select 1000 101X by default.

16.4 HOST WRITE COMMANDS

Some host commands include one or more data bytes written

to the LM8322. Figure 16 shows a SET_KEY_SIZE com-

mand, which consists of an address byte, a command byte,

and one data byte. The first byte is composed of a 7-bit slave

address in bits 7:1 and a direction bit in bit 0. The state of the

direction bit is 0 on writes from the host to the slave and 1 on

reads from the slave to the host.

The second byte sends the command. The SET_KEY_SIZE

command is 0x90.

The third byte send the data, in this case specifying the num-

ber of rows and columns for the keypad.

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