MC34921 Datasheet, PDF (27/36 Pages) List of Unclassifed Manufacturers – Configurable Motor Driver IC with Power Supplies

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
A/D Converter Input Structure
The input impedance of a selected channel is an RC
circuit. As shown in Figure 13, the input impedance of the
selected channel is a resistor connected to the sample and
hold (S/H) capacitor. The sample and hold time is 15 * t_SCLK,
or a minimum of 0.9285 µs. The tolerances of the internal S/
H components are such that a time constant is about 93.6 ns.
Therefore to achieve a proper level on the S/H capacitor
(5*93.6 ns) = 0.468 µs, minimum, is required to satisfy
internal component time constants. This only allows 460 ns
for external time constants. Therefore, the maximum source
impedance of the circuit driving the selected A/D channel is
7.8 kΩ when the SCLK speed matches the speed
configuration bit.
SCLK
Channel Select Logic
AN0/ANALOGOUT_A
AN1/ANALOGOUT_B
C < 11.7 pF
hold
AN2/ANALOGIN_A
S/ H
AN3/ANALOGIN_B
A
I/ V
A
B
I/ V
B
Multiplexer R_DS(on) < 8.0 kΩ
Figure 13. A/D Converter Input Structure
ADC Output Addressing
The return word to the digital subsystem also contains two
conversion done bits, A/Ddone1 and A/Ddone2 (refer to
Table 8). The A/Ddone1 bit is used for single conversions
and the first conversion when input pairs are selected. The
A/Ddone2 bit is used for the second conversion when input
pairs are selected. A zero to one transition of these bits on
successive return words indicates that a conversion cycle is
complete and the data sent in that return word is valid for that
conversion. The conversion done bits will only be asserted for
one serial frame, although the data may be valid for multiple
frames. The A/Dr bits indicate which A/D input is being
reported when data is valid (refer to Table 9).
Table 8. A/D Done Bits
A/Ddone1 A/Ddone2 Return Data and A/Dr Value
Rising
0
Valid for single or first conversion
0
Rising
Valid for second conversion
0
0
Don’t care
Table 9. A/D Channel of Current A/D Data
NORMAL
Output
Frame
Bit 4
NORMAL
Output
Frame
Bit 5
NORMAL
Output
Frame
Bit 6
A/D Value Reported
A /Dr0
A /Dr1
A /Dr2
0
0
1
A
1
0
1
B
0
1
1
A
1
1
1
B
0
0
0
AN0/ANALOGOUT_A
1
0
0
AN1/ANALOGOUT_B
0
1
0
AN2/ANALOGIN_A
(direct input)
1
1
0
AN3/ANALOGIN_B
(direct input)
Analog Integrated Circuit Device Data
Freescale Semiconductor
34921
27