Z89175 Zilog., Z89175 Datasheet - Page 56

• Manufacturer Part Number: Z89175
• Description: Voice Processing Controllers
• Manufacturer: Zilog.

Z89175/Z89176
Voice Processing Controllers
A/D CONVERTER (ADC)
Analog to Digital Converter
The A/D converter is an 8-bit half flash converter which
uses two reference resistor ladders for its upper four bits
(MSBs) and lower four bits (LSBs) conversion (Figure 41).
Two reference voltage pins, VREF+ (High) and VREF-
(Low), are provided for external reference voltage sup-
plies. During the sampling period, the converter is auto-ze-
Conversion begins by writing to the appropriate bit in the
Analog Control Register (ACR). The start commands are
implemented in such a way as to begin a conversion at any
time. If a conversion is in progress and a new start com-
mand is received, then the conversion in progress is abort-
ed and a new conversion initiated. This allows the pro-
grammed values to be changed without affecting a
conversion in progress. The new values take effect only af-
ter a new start command is received.
56
AN IN
Sample
Auto Zero m
Flash
4-Bit
Latch 4 MSB
Latch 4 LSB
Figure 40. A/D Converter
P R E L I M I N A R Y
4 MSB
4-Bit
DAC
roed before starting the conversion time depending on the
external clock frequency and the selection of the A/D sam-
pling rate (Figure 40). The sampling rates are 10 or 16 kHz
(XTAL = 20.48 MHz) in order to provide oversampling. The
rates are software controlled by the ACR (DSP External
Register 6). Timer2 supports the ADC. The maximum con-
version time is 2 s.
The ADC can be disabled (for low power) or enabled by an
analog Control Register bit.
Though the ADC functions for a smaller input voltage and
voltage reference, the noise and offsets remain constant
over the specified electrical range. The errors of the con-
verter will increase and the conversion time can also take
slightly longer due to smaller input signals.
+
–
Bits 9-2 Register 12 of DSP
4 LSB
Flash
4-Bit
Auto Zero
DS97TAD0100
Zilog