Digital Kernel; Digital Signal Processor (U2); Programmable Logic Device - Fluke 39 Service Manual

39/41B
Service Manual

2-25. Digital Kernel

The digital kernel consists of a digital signal processor, a programmable logic device
(PLD), ROM, and RAM. It processes the input signals from the a/d converters, computes
the values and waveforms, and stores the data in RAM for the display.

2-26. Digital Signal Processor (U2)

The DSP56002 processor (DSP) has full control of all hardware in the Tester. It controls
a/d sampling, computations, serial interface, user interface, and the display.
The DSP has several I/O ports that can be configured in a number of different ways. Two
of these ports are for serial communications. The Fluke 41B makes use of one serial port
for RS-232 communication to printers or PCs over a special optical interface and cable.
The second serial port is used by both models to communicate to the two a/d converters
(U14 and U15) and the serial EEPROM (U22). The other I/O ports are used for various
control and sensing: keypad interface, power control, low battery sense, and memory
bank switching.
This DSP, unlike most common microcontrollers, is optimized for calculations instead of
control. The data bus is 24 bits wide. All internal data registers are 24 bits, except two
accumulators which are 56 bits wide.
There are three 64K address spaces that share the same external address and data bus.
The three address spaces are Program, X-data, and Y-data. Code can only be executed
from the program space, but data can be stored and retrieved from all three address
spaces. The control lines DS*, PS*, and X/Y* from the DSP, control which of these
three address spaces are accessed. Although this may sound a bit complex with the three
address spaces, you could think of the PS* and X/Y* signals as two more address lines
(A16 and A17)

2-27. Programmable Logic Device

A Programmable Logic Device (PLD) replaces several generic parts that usually
consume more board space and power. Much like a ROM, these parts can be
programmed into an almost limitless combination of circuits. This makes them easy to
use, but sometimes hard to debug, since they look like a black box from the outside. In
the case of the Fluke 39/41B, U3 is broken into six relatively simple sections. Several of
the sections share common inputs. Figure 2-2 is a block diagram of the internal circuits
of the PLD.
2-28. Keypad Interrupts
The four row signals (R0-3) from the keypad are ANDed together. If any of the signals
drop below a valid high state, IRQA* is driven low. This generates an interrupt to the
DSP so that the software does not have to continually scan the keypad inputs until an
interrupt is detected.
2-10