TLC545C, TLC545I, TLC546C, TLC546I

8-BIT ANALOG-TO-DIGITAL CONVERTERS

WITH SERIAL CONTROL AND 19 INPUTS

SLAS066B – DECEMBER 1985 – REVISED OCTOBER 1996

11

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

It is possible to connect SYSTEM CLOCK and I/O CLOCK together in special situations in which controlling circuitry

points must be minimized. In this case, the following special points must be considered in addition to the requirements

of the normal control sequence previously described.

1.

The first two clocks are required for this device to recognize CS is at a valid low level when the common clock

signal is used as an I/O CLOCK. When CS is recognized by the device to be at a high level, the common clock

signal is used for the conversion clock also.

2.

A low CS must be recognized before the I/O CLOCK can shift in an analog channel address. The device

recognizes a CS transition when the SYSTEM CLOCK terminal receives two positive edges and then a

negative edge. For this reason, after a CS negative edge, the first two clock cycles do not shift in the address.

Also, upon shifting in the address, CS must be raised after the eighth valid (10 total) I/O CLOCK. Otherwise,

additional common clock cycles are recognized as I/O CLOCKS and shift in an erroneous address.

For certain applications, such as strobing applications, it is necessary to start conversion at a specific point in time.

This device accommodates these applications. Although the on-chip sample and hold begins sampling upon the

negative edge of the fourth valid I/O clock cycle, the hold function is not initiated until the negative edge of the eighth

valid I/O clock cycle. Thus, the control circuitry can leave the I/O clock signal in its high state during the eighth valid

I/O clock cycle, until the moment at which the analog signal must be converted. The TLC545/546 continues sampling

the analog input until the eighth valid falling edge of the I/O clock. The control circuitry or software must then

immediately lower the I/O clock signal to initiate the hold function at the desired point in time and to start conversion.