Z89135 Datasheet, PDF (52/66 Pages) Zilog, Inc. – Low-Cost DTAD Controller

Z89135/136 (ROMless)
Low-Cost DTAD Controller
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. Two refer-
ence voltage pins, V_REF+ (High) and V_REF- (Low), are pro-
vided for external reference voltage supplies. During the
sampling period, the converter is auto-zeroed before start-
ing the conversion time depending on the external clock
frequency and the selection of the A/D sampling rate. The
Zilog
sampling rates are in the order of 8, 10, 16, 64, or 128 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 µs.
AN IN
4-Bit
Flash
4-Bit
DAC
–
4-Bit
+
Flash
Sample
Auto Zero m
Latch 4 MSB
Latch 4 LSB
4 MSB
4 LSB ^{Auto Zero}
Bits 9-2 Register 12 of DSP
Figure 39. A/D Converter
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.
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 may also take
slightly longer due to smaller input signals.
1-52
PRELIMINARY
DS97TAD0300