Motorola ColdFire MCF5281 - Manual (Page 594)

Queued Analog-to-Digital Converter (QADC)
28-56 Freescale Semiconductor
NOTE
Although the result RAM can be written, some write operations, like bit
manipulation, may not operate as expected because the hardware cannot
access a true 16-bit value.
While there is only one result word table, the half-word (16-bit) data can be accessed in three different data
formats:
• Right justified with 0s in the higher order unused bits
• Left justified with the most significant bit inverted to form a sign bit, and 0s in the unused lower
order bits
• Left justified with 0s in the lower order unused bits
The left justified, signed format corresponds to a half-scale, offset binary, two’s complement data format.
The address used to read the result table determines the data alignment format. All write operations to the
result word table are right justified.
28.9 Signal Connection Considerations
The QADC requires accurate, noise-free input signals for proper operation. This section discusses the
design of external circuitry to maximize QADC performance.
28.9.1 Analog Reference Signals
No A/D converter can be more accurate than its analog reference. Any noise in the reference can result in
at least that much error in a conversion. The reference for the QADC, supplied by signals V
RH
and V
RL
,
should be low-pass filtered from its source to obtain a noise-free, clean signal. In many cases, simple
capacitive bypassing may suffice. In extreme cases, inductors or ferrite beads may be necessary if noise or
RF energy is present. External resistance may introduce error in this architecture under certain conditions.
Any series devices in the filter network should contain a minimum amount of DC resistance.
For accurate conversion results, the analog reference voltages must be within the limits defined by V
DDA
and V
SSA
, as explained in this subsection.
28.9.2 Analog Power Signals
The analog supply signals (V
DDA
and V
SSA
) define the limits of the analog reference voltages (V
RH
and
V
RL
) and of the analog multiplexer inputs. Figure 28-44 is a diagram of the analog input circuitry.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3