LM40 datasheet(11/27 Pages) NSC

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1.0 Functional Description (Continued)

that begin with the label Mout_ depict a drive by the master.

Signal labels that begin with the label Slv_ depict the drive by

the LM40. All other signals show what would be seen when

probing SWD for a particular function (e.g. "Master Wr 0" is

the Master transmitting a Data Bit with the value of 0).

1.2.1 Bus Inactive

The bus is inactive when the SWD signal is high for a period

of at least t

INACT. The bus is inactive between each "bit

signal".

1.2.2 Data Bit 0 and 1

All Data Bit signal transfers are started by the master. A Data

Bit 0 is indicated by a "short" pulse; a Data Bit 1 is indicated

by a longer pulse. The direction of the bit is relative to the

master, as follows:

•

Data Write - a Data Bit transferred from the master to the

LM40.

•

Data Read - a Data Bit transferred from the LM40 to the

master.

A master must monitor the bus as inactive before starting a

Data Bit (Read or Write).

A master initiates a data write by driving the bus active (low

level) for the period that matches the data value (t

Mtr0 or tMtr1

for a write of "0" or "1", respectively). The LM40 will detect

that the SWD becomes active within a period of t

SFEdet, and

will start measuring the duration that the SWD is active in

order to detect the data value.

A master initiates a data read by driving the bus for a period

of t

Mtr0. The LM40 will detect that the SWD becomes active

within a period of t

SFEdet. For a data read of "0", the LM40

will not drive the SWD. For a data read of "1" the LM40 will

start within t

SFEdet to drive the SWD low for a period of

SLout1. Both master and LM40 must monitor the time at

which the bus becomes inactive to identify a data read of "0"

or "1".

During each Data Bit, both the master and all the LM40s

must monitor the bus (the master for Attention Request and

Reset; the LM40s for Start Bit, Attention Request and Reset)

by measuring the time SWD is active (low). If a Start Bit,

Attention Requests or Reset "bit signal" is detected, the

current "bit signal" is not treated as a Data Bit.

Note that the bit rate of the protocol varies depending on the

data transferred. Thus, the LM40 has a value of "0" in

reserved or unused register bits for bus bandwidth efficiency.

1.2.3 Start Bit

A master must monitor the bus as inactive before beginning

a Start Bit.

The master uses a Start Bit to indicate the beginning of a

transfer. LM40s will monitor for Start Bits all the time, to allow

synchronization of transactions with the master. If a Start Bit

occurs in the middle of a transaction, the LM40 being ad-

dressed will abort the current transaction. In this case the

transaction is not "completed" by the LM40 (see Section 1.3

"SensorPath Bus Transactions").

During each Start Bit, both the master and all the LM40s

must monitor the bus for Attention Request and Reset, by

measuring the time SWD is active (low). If an Attention

Request or Reset condition is detected, the current "bit

signal" is not treated as a Start Bit. The master may attempt

to send the Start Bit at a later time.

1.2.4 Attention Request

The LM40 may initiate an Attention Request when the Sen-

sorPath bus is inactive.

Note that a Data Bit, or Start Bit, from the master may start

simultaneously with an Attention Request from the LM40. In

addition, two LM40s may start an Attention Request simul-

taneously. Due to its length, the Attention Request has pri-

ority over any other "bit signal", except Reset. Conflict with

Data Bits and Start Bits are detected by all the devices, to

allow the bits to be ignored and re-issued by their originator.

The LM40 will either check to see that the bus is inactive

before starting an Attention Request, or start the Attention

Request within the t

SFEdet time interval after SWD becomes

active. The LM40 will drive the signal low for t

SLoutA time.

After this, both the master and the LM40 must monitor the

bus for a Reset Condition. If a Reset condition is detected,

the current "bit signal" is not treated as an Attention Request.

After Reset, an Attention Request can not be sent before the

master has sent 14 Data Bits on the bus. See Section 1.3.5

for further details on Attention Request generation.

1.2.5 Bus Reset

The LM40 issues a Reset at power up. The master must also

generate a Bus Reset at power-up for at least the minimum

reset time, it must not rely on the LM40. SensorPath puts no

limitation on the maximum reset time of the master. Follow-

ing a Bus Reset, the LM40 may generate an Attention Re-

quest only after the master has sent 14 Data Bits on the bus.

See Section 1.3.5 for further details on Attention Request

generation.

1.3 SensorPath BUS TRANSACTIONS

SensorPath is designed to work with a single master and up

to seven slave devices. Each slave has a unique address.

The LM40 supports up to 2 device addresses that are se-

lected by the state of the address pin ADD. The Register Set

of the LM40 is defined in Section 2.0.

1.3.1 Bus Reset Operation

A Bus Reset Operation is global on the bus and affects only

the communication interface of all the devices connected to

it. The Bus Reset operation does not affect either the con-

tents of the device registers, or device operation, to the

extent defined in LM40 Register Set, see Section 2.0.

The Bus Reset operation is performed by generating a Reset

signal on the bus. The master must apply Reset after power-

up, and before it starts operation. The Reset signal end will

be monitored by all the LM40s on the bus.

After the Reset Signal the SensorPath specification requires

that the master send a sequence of 8 Data Bits with a value

of "0", without a preceding Start Bit. This is required to

enable slaves that "train" their clocks to the bit timing. The

LM40 does not require nor does it support clock training.

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