
1996 Feb 21
18
Philips Semiconductors
Objective specification
Microcontrollers for digital auto-sync and
VST TV controller applications
PCE84C486; PCE84C487
7.
For the DAC = 1100 case
If COMP = 1; then the analog input voltage is still
greater than V
ref
and therefore the digital input to the
DAC needs to be increased again. Set the input to the
DAC to 1110.
If COMP = 0; then the analog input voltage is now less
than V
ref
and therefore the digital input to the DAC
needs to be decreased. Set the input to the DAC to
1010
For the DAC = 0100 case
If COMP = 1; then the analog input voltage is now
greater than V
ref
and therefore the digital input to the
DAC needs to be increased. Set the input to the DAC
to 0110.
If COMP = 0; then the analog input voltage is still lower
than V
ref
and therefore the digital input to the DAC
needs to be decreased again. Set the input to the DAC
to 0010.
8.
9.
The operations detailed in 6, 7 and 8 above are
repeated and each time the digital input to the DAC is
changed accordingly; as dictated by the state of the
COMP bit. The complete process is shown in Fig.16.
Each time the DAC input is changed the number of
values which the analog input can take is reduced by
half. In this manner the actual analog value is honed
into. The value of the analog input (V
A
) is determined
using Equation (4):
(4)
As the conversion time of each compare operation is
greater than 6
μ
s but less than 9
μ
s; a NOP instruction is
recommended to be used in between the instructions that
change the value of V
ref
; select the ADC channel and read
the COMP bit.
V
A
V
16
----------
DAC value
1
+
(
)
×
=
Fig.15 Block diagram of 2 channel ADC.
handbook, full pagewidth
4-BIT DAC
COMPARATOR
EN
DAC3
DAC2
DAC1
DAC0
ADCE1
ADCE2
ADCS1
ADCS0
MGD263
ADC enable selection
DAC value selection
ENABLE
SELECTOR
ADC
CHANNEL
SELECTOR
DP12/ADC2
DP11/ADC1
Vref
DERIVATIVE PORT
SELECTOR
EN1
EN2
‘MOV A, D20’
instruction
to read COMP bit
COMP bit
Internal bus
Channel selection
+