TLC548C, TLC548I, TLC549C, TLC549I
8-BIT ANALOG-TO-DIGITAL CONVERTERS
WITH SERIAL CONTROL
SLAS067C – NOVEMBER 1983 – REVISED SEPTEMBER 1996
PRINCIPLES OF OPERATION
The TLC548 and TLC549 are each complete data acquisition systems on a single chip. Each contains an internal
system clock, sample-and-hold function, 8-bit A/D converter, data register, and control logic circuitry. For flexibility
and access speed, there are two control inputs: I/O CLOCK and chip select (CS). These control inputs and a
TTL-compatible 3-state output facilitate serial communications with a microprocessor or minicomputer. A conversion
can be completed in 17
µs
or less, while complete input-conversion-output cycles can be repeated in 22
µs
for the
TLC548 and in 25
µs
for the TLC549.
The internal system clock and I/O CLOCK are used independently and do not require any special speed or phase
relationships between them. This independence simplifies the hardware and software control tasks for the device.
Due to this independence and the internal generation of the system clock, the control hardware and software need
only be concerned with reading the previous conversion result and starting the conversion by using the I/O clock. In
this manner, the internal system clock drives the “conversion crunching” circuitry so that the control hardware and
software need not be concerned with this task.
When CS is high, DATA OUT is in a high-impedance condition and I/O CLOCK is disabled. This CS control function
allows I/O CLOCK to share the same control logic point with its counterpart terminal when additional TLC548 and
TLC549 devices are used. This also serves to minimize the required control logic terminals when using multiple
TLC548 and TLC549 devices.
The control sequence has been designed to minimize the time and effort required to initiate conversion and obtain
the conversion result. A normal control sequence is:
1. CS is brought low. To minimize errors caused by noise at CS, the internal circuitry waits for two rising edges
and then a falling edge of the internal system clock after a CS↓ before the transition is recognized. However,
upon a CS rising edge, DATA OUT goes to a high-impedance state within the specified t
dis
even though the
rest of the integrated circuitry does not recognize the transition until the specified t
su(CS)
has elapsed. This
technique protects the device against noise when used in a noisy environment. The most significant bit (MSB)
of the previous conversion result initially appears on DATA OUT when CS goes low.
2. The falling edges of the first four I/O CLOCK cycles shift out the second, third, fourth, and fifth most significant
bits of the previous conversion result. The on-chip sample-and-hold function begins sampling the analog
input after the fourth high-to-low transition of I/O CLOCK. The sampling operation basically involves the
charging of internal capacitors to the level of the analog input voltage.
3. Three more I/O CLOCK cycles are then applied to the I/O CLOCK terminal and the sixth, seventh, and eighth
conversion bits are shifted out on the falling edges of these clock cycles.
4. The final (the eighth) clock cycle is applied to I/O CLOCK. The on-chip sample-and-hold function begins the
hold operation upon the high-to-low transition of this clock cycle. The hold function continues for the next four
internal system clock cycles, after which the holding function terminates and the conversion is performed
during the next 32 system clock cycles, giving a total of 36 cycles. After the eighth I/O CLOCK cycle, CS must
go high or the I/O clock must remain low for at least 36 internal system clock cycles to allow for the completion
of the hold and conversion functions. CS can be kept low during periods of multiple conversion. When
keeping CS low during periods of multiple conversion, special care must be exercised to prevent noise
glitches on the I/O CLOCK line. If glitches occur on I/O CLOCK, the I/O sequence between the
microprocessor/controller and the device loses synchronization. When CS is taken high, it must remain high
until the end of conversion. Otherwise, a valid high-to-low transition of CS causes a reset condition, which
aborts the conversion in progress.
A new conversion may be started and the ongoing conversion simultaneously aborted by performing steps 1 through
4 before the 36 internal system clock cycles occur. Such action yields the conversion result of the previous conversion
and not the ongoing conversion.
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