SC1100 datasheet(97/348 Pages) NSC

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SuperI/O Module (Continued)

4.7

ACCESS.BUS INTERFACE

The SC1100 has two ACCESS.bus (ACB) controllers. ACB

is a two-wire synchronous serial interface compatible with

the ACCESS.bus physical layer, Intel's SMBus, and Phil-

ips’ I2C. The ACB can be configured as a bus master or

slave, and can maintain bidirectional communication with

both multiple master and slave devices. As a slave device,

the ACB may issue a request to become the bus master.

The ACB allows easy interfacing to a wide range of low-

cost memories and I/O devices, including: EEPROMs,

SRAMs, timers, ADC, DAC, clock chips and peripheral

drivers.

The ACCESS.bus protocol uses a two-wire interface for

bidirectional communication between the ICs connected to

the bus. The two interface lines are the Serial Data Line

(AB1D and AB2D) and the Serial Clock Line (AB1C and

AB2C). (Here after referred to as ABD and ABC unless oth-

erwise specified.)These lines should be connected to a

positive supply via an internal or external pull-up resistor,

and remain high even when the bus is idle.

Each IC has a unique address and can operate as a trans-

mitter or a receiver (though some peripherals are only

receivers).

During data transactions, the master device initiates the

transaction, generates the clock signal and terminates the

transaction. For example, when the ACB initiates a data

transaction with an attached ACCESS.bus compliant

peripheral, the ACB becomes the master. When the

peripheral responds and transmits data to the ACB, their

master/slave (data transaction initiator and clock genera-

tor) relationship is unchanged, even though their transmit-

ter/receiver functions are reversed.

This section describes the general ACB functional block. A

device may include a different implementation. For device

specific implementation, see Section 4.4.2.4 "LDN 05h and

06h - ACCESS.bus Ports 1 and 2" on page 78.

4.7.1

Data Transactions

One data bit is transferred during each clock pulse. Data is

sampled during the high state of the serial clock (ABC).

Consequently, throughout the clock’s high period, the data

should remain stable (see Figure 4-13). Any changes on

the ABD line during the high state of the ABC and in the

middle of a transaction aborts the current transaction. New

data should be sent during the low ABC state. This protocol

permits a single data line to transfer both command/control

information and data, using the synchronous serial clock.

Each data transaction is composed of a Start Condition, a

number of byte transfers (set by the software) and a Stop

Condition to terminate the transaction. Each byte is trans-

ferred with the most significant bit first, and after each byte

(8 bits), an Acknowledge signal must follow. The following

sections provide further details of this process.

During each clock cycle, the slave can stall the master

while it handles the previous data or prepares new data.

This can be done for each bit transferred, or on a byte

boundary, by the slave holding ABC low to extend the

clock-low period. Typically, slaves extend the first clock

cycle of a transfer if a byte read has not yet been stored, or

if the next byte to be transmitted is not yet ready. Some

microcontrollers,

with

limited

hardware

support

for

ACCESS.bus, extend the access after each bit, thus allow-

ing the software to handle this bit.

Figure 4-13. Bit Transfer

4.7.2

Start and Stop Conditions

The ACCESS.bus master generates Start and Stop Condi-

tions (control codes). After a Start Condition is generated,

the bus is considered busy and retains this status for a cer-

tain time after a Stop Condition is generated. A high-to-low

transition of the data line (ABD) while the clock (ABC) is

high indicates a Start Condition. A low-to-high transition of

the ABD line while the ABC is high indicates a Stop Condi-

tion (Figure 4-14).

In addition to the first Start Condition, a repeated Start

Condition can be generated in the middle of a transaction.

This allows another device to be accessed, or a change in

the direction of data transfer.

Figure 4-14. Start and Stop Conditions

ABD

ABC

Data Line

Stable:

Data Valid

Change

of Data

Allowed

ABD

ABC

Start

Condition

Stop

Condition

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