TLV2553

SLAS354B – SEPTEMBER 2001 – REVISED SEPTEMBER 2002

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PRINCIPLES OF OPERATION

data output length (continued)

With bits D3 and D2 set to 01, the 8-bit data-length mode is selected, which allows fast communication with 8-bit

serial interfaces. In the 8-bit mode, the result of the current conversion is output as an 8-bit serial data stream

during the next I/O cycle. The current I/O cycle must be exactly eight bits long to maintain synchronization, even

when this means corrupting the output data from the previous conversion. The four LSBs of the conversion

result are truncated and discarded. The current conversion is started immediately after the eighth falling edge

of the current I/O cycle.

Since the D3 and D2 register settings take effect on the I/O cycle when the data length is programmed, there

can be a conflict with the previous cycle if the data-word length was changed. This may occur when the data

format is selected to be least significant bit first, since at the time the data length change becomes effective (six

rising edges of I/O CLOCK), the previous conversion result has already started shifting out. In actual operation,

when different data lengths are required within an application and the data length is changed between two

conversions, no more than one conversion result can be corrupted and only when it is shifted out in LSB-first

format.

LSB out first

D1 in the CFGR1 controls the direction of the output (binary) data transfer. When D1 is reset to 0, the conversion

result is shifted out MSB first. When set to 1, the data is shifted out LSB first. Selection of MSB first or LSB first

always affects the next I/O cycle and not the current I/O cycle. When changing from one data direction to

another, the current I/O cycle is never disrupted.

bipolar output format

D0 in the CFGR1 controls the binary data format used to represent the conversion result. When D0 is cleared

to 0, the conversion result is represented as unipolar (unsigned binary) data. Nominally, the conversion result

When D0 is set to 1, the conversion result is represented as bipolar (signed binary) data. Nominally, conversion

as the sign bit. The bipolar data format is related to the unipolar format in that the MSBs are always each other’s

complement.

Selection of the unipolar or bipolar format always affects the current conversion cycle, and the result is output

during the next I/O cycle. When changing between unipolar and bipolar formats, the data output during the

current I/O cycle is not affected.

reference

An external reference can be used through two reference input pins, REF+ and REF–. The voltage levels

applied to these pins establish the upper and lower limits of the analog inputs to produce a full-scale and

zero-scale reading respectively. The values of REF+, REF–, and the analog input should not exceed the positive

supply or be lower than GND consistent with the specified absolute maximum ratings. The digital output is at

full scale when the input signal is equal to or higher than REF+ and at zero when the input signal is equal to or

lower than REF–.