83C145 Datasheet, PDF (14/40 Pages) NXP Semiconductors – Microcontrollers for TV and video MTV

Philips Semiconductors
Microcontrollers for TV and video (MTV)
Product specification
83C145; 83C845
83C055; 87C055
12 SOFTWARE ANALOG-TO-DIGITAL FACILITY
Figure 6 shows the software analog-to-digital facility block diagram. The block includes Special Function Register SAD.
12.1 Special Function Register SAD
Table 12 Special Function Register SAD format (address D8H)
7
6
5
VHi
CH1
CH0
4
3
St
SAD3
2
SAD2
1
SAD1
0
SAD0
Table 13 Description of SAD bits
BIT
7
6
5
4
3 to 0
SYMBOL
DESCRIPTION
VHi
The comparator output bit; bit addressable.
CH1
The channel field controls which pin, if any, is connected to this facility; see Table 14.
CH0
St
The St bit should be written as a logic 1 in order to initiate a voltage comparison.
SAD3 to SAD0 4 LSBs of the SAD register.
12.2 Software ADC operation
Port pins P1.0/ADI0 to P1.2/ADI2 can be alternately
selected as inputs of a linear voltage comparator. The
other input of the comparator is connected to a 4-bit DAC.
This DAC is controlled by bits SAD3 to SAD0 and
produces a reference voltage:
nominally 0.15625 to 4.84375 V in increments of
0.3125 V.
The output of the comparator (HIGH or LOW) can be read
by the program as the MSB of the SAD register i.e. bit VHi.
After writing St = 1, the program should include intervening
instructions totalling at least 6 machine cycles (72 clock
periods or 6 µs at 12 MHz), before the instruction that
accesses and tests VHi.
Table 14 Pin selection: P1.n/ADIn
CH1
0
0
1
1
CH0
0
1
0
1
P1.n/ADIn⁽¹⁾
none
P1.0/ADI0
P1.1/ADI1
P1.2/ADI2
Note
1. Port 1 has open-drain drivers which will not materially
affect an analog voltage as long as any and all pins
used for software analog-to-digital measurement have
corresponding logic 1s in the port register; n = 0, 1, 2.
1996 Mar 22
14