L6238 datasheet(28/35 Pages) STMICROELECTRONICS

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Figure 30 is the timing diagram for writing to the

serial port. This diagram indicates the typical

waveforms at the serial port and how they relate

to one another when the PORT DISABLE pin is

used. Two consecutive write cycles with key tim-

ing parameters are illustrated.

To initiate the write cycle, the STROBE and R/W

signals are first brought low. After a minimum set-

up time, Tos, the PORT DISABLE pin is set low.

The clocking of the data can begin after a mini-

mum settling time, Tsettle has passed. The data is

clocked into the register on the falling edge of the

PORT CLOCK. After the 16th clock cycle and

wait time Tcsw a strobe signal causes the data to

be transferred to the 16-bit latch.

Additional timing parameters that are relevant

concern the timing of the clock signal relative to

the data stream. The time Tds is the data set up

time, where the data must be stable before the

falling edge of the clock. The Data Hold time, Tdh,

is the minimum time that the data must be valid

after the rising edge of the clock pulse.

The waveforms associated with reading from the

serial port are similar to the write mode. The main

difference is in the timing of the strobe pulse.

Since there is a single port for both read and

write, the strobe signal, in conjunction with the

R/W signal insures proper data stream flow.

Referring to figure 31, the read mode is initiated

by first asserting the R/W line high, while holding

the strobe line low. The PORT DISABLE pin is

then brought low. A pulse is now sent to the

strobe pin that transfers the data on the Status

Bus to the Shift Register. The falling edge of the

strobe cannot occur earlier than the minimum set-

tling time, Tsettle. The data is shifted out on the

Figure 31: Read Timing Diagram.

Figure 30: Write Timing Diagram.

L6238

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