ADC0804LCN/NOPB National Semiconductor, ADC0804LCN/NOPB Datasheet - Page 35

• Manufacturer Part Number: ADC0804LCN/NOPB
• Description: IC ADC 8BIT MPU COMPAT 20-DIP
• Manufacturer: National Semiconductor

Specifications of ADC0804LCN/NOPB

• Number Of Bits: 8
• Number Of Converters: 1
• Power Dissipation (max): 875mW
• Voltage Supply Source: Single Supply
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *ADC0804LCN; *ADC0804LCN/NOPB; ADC0804

Functional Description
A flow chart for the zeroing subroutine is shown in Figure 20 .
It must be noted that the ADC0801 series will output an all
zero code when it converts a negative input [V
Also, a logic inversion exists as all of the I/O ports are
buffered with inverting gates.
Basically, if the data read is zero, the differential output
voltage is negative, so a bit in Port B is cleared to pull V
more negative which will make the output more positive for
the next conversion. If the data read is not zero, the output
voltage is positive so a bit in Port B is set to make V
positive and the output more negative. This continues for 8
approximations and the differential output eventually con-
verges to within 5 mV of zero.
The actual program is given in Figure 21 . All addresses used
are compatible with the BLC 80/10 microcomputer system.
In particular:
5.3 Multiple A/D Converters in a Z-80 Interrupt
Driven Mode
In data acquisition systems where more than one A/D con-
verter (or other peripheral device) will be interrupting pro-
gram execution of a microprocessor, there is obviously a
Port A and the ADC0801 are at port address E4
Port B is at port address E5
Port C is at port address E6
PPI control word port is at port address E7
Program Counter automatically goes to ADDR:3C3D upon
acknowledgement of an interrupt from the ADC0801
FIGURE 19. Microprocessor Interface Circuitry for Differential Preamp
(Continued)
IN
(−)
V
X
IN
more
(+)].
X
35
need for the CPU to determine which device requires servic-
ing. Figure 22 and the accompanying software is a method
of determining which of 7 ADC0801 converters has com-
pleted a conversion (INTR asserted) and is requesting an
interrupt. This circuit allows starting the A/D converters in
any sequence, but will input and store valid data from the
converters with a priority sequence of A/D 1 being read first,
A/D 2 second, etc., through A/D 7 which would have the
lowest priority for data being read. Only the converters
whose INT is asserted will be read.
The key to decoding circuitry is the DM74LS373, 8-bit D type
flip-flop. When the Z-80 acknowledges the interrupt, the
program is vectored to a data input Z-80 subroutine. This
subroutine will read a peripheral status word from the
DM74LS373 which contains the logic state of the INTR
outputs of all the converters. Each converter which initiates
an interrupt will place a logic “0” in a unique bit position in the
status word and the subroutine will determine the identity of
the converter and execute a data read. An identifier word
(which indicates which A/D the data came from) is stored in
the next sequential memory location above the location of
the data so the program can keep track of the identity of the
data entered.
DS005671-92
www.national.com