AD7878
INTERNAL FIFO MEMORY
INTERNAL COMPARATOR TIMING
The internal FIFO memory of the AD7878 consists of eight
memory locations. Each word in memory contains 13 bits of
information—12 bits of data from the conversion result and one
additional bit which contains information as to whether the 12-
bit result is out of range or not. A block diagram of the AD7878
FIFO architecture is shown in Figure 3.
The ADC clock, which is applied to CLK IN, controls the suc-
cessive approximation A/D conversion process. This clock is
internally divided by four to yield a bit trial cycle time of 500 ns
min (CLK IN = 8 MHz clock). Each bit decision occurs 25 ns
after the rising edge of this divided clock. The bit decision is
latched by the rising edge of an internal comparator strobe sig-
nal. There are 12-bit decisions, as in a normal successive ap-
proximation routine, and one extra decision that checks if the
input sample is out of range. In a normal successive approxima-
tion A/D converter, reading data from the device during conver-
sion can upset the conversion in progress. This is due to on-chip
transients, generated by charging or discharging the databus,
concurrent with a bit decision. The scheme outlined below and
shown in Figure 4 describes how the AD7878 overcomes this
problem.
The internal comparator strobe on the AD7878 is gated with
both DMRD and DMWR so that if a read or write operation
occurs when a bit decision is about to be made, the bit decision
point is deferred by one CLK IN cycle. In other words, if
DMRD or DMWR goes low (with CS low) at any time during
the CLK IN low time immediately prior to the comparator
strobing edge (tLOW of Figure 4), the bit trial is suspended for a
clock cycle. This makes sure that the bit decision is latched at a
time when the AD7878 is not attempting to charge or discharge
the data bus, thereby ensuring that no spurious transients occur
internally near a bit decision point.
Figure 3. Internal FIFO Architecture
The conversion result is gathered in the successive approxima-
tion register (SAR) during conversion. At the end of conversion
this result is transferred to the FIFO memory. The FIFO ad-
dress pointer always points to the top of memory, which is the
uppermost location containing valid data. The pointer is incre-
mented after each conversion. A read operation from the FIFO
memory accesses data from the bottom of the FIFO, Location 0.
On completion of the read operation, each data word moves
down one location and the address pointer is decremented by
one. Therefore, each conversion result from the SAR enters at
the top of memory, propagates down with successive reads until
it reaches Location 0 from where it can be accessed by a micro-
processor read operation.
The decision point slippage mechanism is shown in Figure 4 for
the MSB decision. Normally, the MSB decision occurs 25 ns
after the fourth rising CLK IN edge after CONVST goes high.
However, in the timing diagram of Figure 4, CS and DMRD or
DMWR are low in the time period tLOW prior to the MSB deci-
sion point on the fourth rising edge. This causes the internal
comparator strobe to be slipped to the fifth rising clock edge.
The AD7878 will again check during a period tLOW prior to this
fifth rising clock edge; and if the CS and DMRD or DMWR are
still low, the bit decision point will be slipped a further clock
cycle.
The transfer of information from the SAR to the FIFO occurs in
synchronization with the AD7878 input clock (CLK IN). The
propagation of data words down the FIFO is also synchronous
with this clock. As a result, a read operation to obtain data from
the FIFO must also be synchronous with CLK IN to avoid
Read/Write conflicts in the FIFO (i.e., reading from FIFO Loca-
tion 0 while it is being updated). This requires that the micro-
processor clock and the AD7878 CLK IN are derived from the
same source.
The conversion time for the ADC normally consists of the 13-
bit trials described above and one extra internal clock cycle during
which data is written from the SAR to the FIFO. For an 8 MHz
input clock this results in a conversion time of 7 µs. However,
the software routine servicing the AD7878 has the potential to
read 16 times from the device during conversion—8 reads from
the FIFO and 8 reads from the status/control register. It also has
the potential to write once to the status/control register. If these
Figure 4. Operational Timing Diagram
–6–
REV. A
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