Z90251 ZILOG [Zilog, Inc.], Z90251 Datasheet - Page 88

• Manufacturer Part Number: Z90251
• Description: eZVision 200 Television Controllers with OSD
• Manufacturer: ZILOG [Zilog, Inc.]

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Analog-to-Digital Converter
The Z90255 is equipped with a 4-bit flash analog-to-digital converter (ADC) that
can be used as either three or four bit configurations. There are four multiplexed
analog-input channels. There are two register addresses, one for 3-bit (Table 70)
ADC (3ADC_DTA: 00h: Bank C), and one for 4-bit (Table 71)
ADC (4ADC_DTA: 01h: Bank F). Because no default is set, system software
must configure the control register for the preferred ADC.
Converted 3-bit data is available as bits 0, 1, and 2 of the 3-bit ADC data register.
Converted 4-bit data is available as bits 0, 1, 2, and 3 of the 4-bit ADC data
register.
Figure 21 illustrates four input pins (P60/ADC3, P61/ADC2, P41/ADC1, and
P62/ADC0) which function as analog-input channels and as digital I/O ports. To
support the analog function, the digital ports must be configured as analog
through software. Analog/digital selection is controlled by bits 4 and 3 of the 3-bit
ADC Data Register, and by bits 5 and 4 of 4-bit ADC Data Register.
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Sampling occurs at one-eighth of an ADC-clock tick. One ADC-clock tick equals
one-half, one-third, or one-quarter of a system-clock (SCLK) tick, as set by
3ADC_DTA(6,5) for 3-bit or 4ADC_DTA (7,6) for 4-bit. If ADC speed bits are
set to 00, the ADC is not operative; this is the default value following POR. If these
bits equal 01, ADC speed is based on one-half of a system-clock tick, SCLK/2. If
these bits equal 10, ADC speed is based on one-third of a system-clock tick,
SCLK/3. If these bits equal 11, ADC speed is based on one-quarter of a system-
clock tick, SCLK/4.
If ADC Input Selection equals 00, ADC0 is selected; this value is the default
following POR.
If ADC Input Selection equals 10, ADC2 is selected.
If ADC Input Selection equals 11, ADC3 is selected.
If ADC Input Selection equals 01, ADC1 is selected.
32 KB Television Controller with OSD
PS001301-0800
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