Fluke PM6690, Service Manual, Page: 93 / 200

Troubleshooting 6-49
The fan is kept at +8.4 V for the first 8.3 minutes. After that
the fan is temperature controlled. The processor reads the
temperature via the I
2
C bus every 10th second. IC U39 mea-
sures the temperature.
The keys on the display board are read over the I
2
C bus. If a
key is pressed, the I
2
C bus circuit U3 notices that and sends an
interrupt to the processor. Check at J13:9; low is interrupt.
The processor then scans the keys via the I
2
C bus to find the
depressed key. See Figure 6-82. During the scanning there
may appear some extra interrupts. This is not an error condi-
tion.
Microprocessor Bus &
Interfaces
The microprocessor bus is divided into two parts with buffers.
The inner part consists of the Flash PROM and the SDRAMs.
Buffers isolate the inner part from long lines in order to make
the SDRAM work safely. The buffers of the 32-bit data bus
are bidirectional and a control signal opens the buffers only
during reads and writes (U56:8, low to open buffers). The di-
rection of the buffers is controlled by the
rdn from the proces-
sor.
The outer part consists of the 32-bit data bus and the 5-bit ad-
dress bus. It connects the processor (U13) to the FPGA (U11),
the GPIB and the USB. See Figure 6-87.
The FPGA connection has 32 data bits, 5 address bits, chip se-
lect,
wrn and rdn . The FPGA is controlled by the processor via
the bus; measurement functions are selected, for instance. The
FPGA is controlled between each measurement or block of
measurements. An interrupt signal from the FPGA is con-
nected to the processor. See Figure 6-89 for a typical timing
diagram.
Figure 6-82
Keyboard interrupt .
Figure 6-83
LCD control signals, oscillogram #6.
LP
RESN
RAN
FRM
D1