AN1816

9

HARDWARE DESIGN

The MC33790 Bus Transceiver is an analogue SmartMOS™ device,

which serves as the physical interface to the two-wire DSI bus. It uses a

combination of pulse length encoded voltage levels for transmit data to

slave nodes and current return signals for receive data at the same time.

Figure 9 MC68HC55 frame and output signals with respect to MC33790 DSIx0 signal

When the MC33790 receives data for transmission to the slave nodes

from the MC68HC55, its bus transmitter circuit converts the 0 to 5 volt

inputs from the MC68HC55’s DSIxF (frame) and DSIxS (signal) to a

voltage level on the output DSIxO which connects to BUS_IN of the first

slave node. The value output on DSIxO of the MC33790 depends on the

values of DSIxF and DSIxS as shown in Table 1 DSIxO Truth Table.

See also Figure 9 MC68HC55 frame and output signals with respect to

MC33790 DSIx0 signal.

When the MC33790 is receiving data from the slave nodes, it samples

the current responses on the rising edge of the final third of bit time. The

current response is then compared against a reference point, which

determines whether a low or high has been returned.

DSIxF (frame)

from MC68HC55

DSIxS (signal)

from MC68HC55

DSIxO

from MC33790

Idle

Idle

High

Low

DSIxF

DSIxS

DSIxR

DSIxO

0

0

Return Data

Low (1.5V)

0

1

Return Data

High (4.5V)

1

0

0

High Impedance

1

1

0

Idle

Table 1 DSIxO Truth Table

Freescale Semiconductor, Inc.

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