Philips PM2519 - Page 33

2.4.2, . Microcomputer
The integrated circuit MAB 8440, one of the MCS-48 family of single-chip microcomputers, forms the basis of
the digital section of the PM 2519. The MAB 8440 has an internal 4k ROM and 128 bytes RAM with address/
data decoding facilities.
In addition to this, the 8440 has 20 quasi-bidirectional I/O ports. Data written to these ports remains
unchanged until written. Each line is able to serve as input or output, or both, even through outputs are
statically latched.
The microcomputer has been designed with an 12C bus to perform data transfer (see 12¢).
spA/P23. : Used as serial output/input (can also be used as input/
outport).
SCL : Bidirectional clock for serial 1/0.
PORT 0 : 8-bit quasi-bidirectional 1/O port used to read the func-
tion switch.
iNT/TO : INT input from the ADC interface to indicate that a com-
plete measurement has been made.
1 : Input pin, testable, Is used to count the pulses coming
from the frequency divider in Hz measurements.
Also used to indicate that the crest factor has been ex-
> ee ceeded,
uP : Crystal input for the internal oscillator.
MAB8440/D021
RESET : Input, used to initialize the microcomputer.
RORT 1 : 8-bit quasi-bidirectional I/O port used to read the func-
tion switch, mode switches and the A-bus.
P10 is used to switch off the battery power supply in the
case of a low battery.
PORT 2 : 4-bit quasi bidirectional 1/O port, P23 is used as serial
output/input.
VDD : Program power supply (0 V) logic 1.
VSS : Circuit earth potential (GND) (—5 V) logic 0.
ST4411
Fig. 2.14, Microcomputer 840720