Troubleshooting The Keypad - Fluke 39 Service Manual

•
SCK* (U14/U15-13 and U3-20) - A complemented version of SCK coming from the
DSP chip.
•
CS_AD* (U14/U15-1 and U3-17) - The chip select signal to the A/Ds which causes
the A/Ds to sample and convert the result.
•
DOUT_VOLTS (U15-12 or U3-13) - The serial data transmitted by the volts A/D
converter.
•
DOUT_AMPS (U14-12 or U3-11) - The serial data transmitted by the amps A/D
converter.
Pins 13 and 11 on U3 should show a slightly attenuated version of pin 12 on U15 and
U14. If this is not the case, verify that the 100Ω resistors (R71/R76) are correct.
When the instrument is collecting data in normal operation mode, the above four
waveforms can be observed with the following timing.
If SCK* and CS_AD* are both correct and present at the A/D but the appropriate
DOUT_VOLTS/DOUT_AMPS is not correct, there is some problem with the front end,
possibly with the A/D converter itself.
The following problems may indicate a defective U3:
•
MS 12 bits of DOUT_VOLTS followed by the MS 12 bits of DOUT_AMPS do not
make it through U3 correctly.
•
SC0 and SC2 (inputs to U3) are correct, but CS_AD* (output from U3) is not
correct.
•
SCK* is not an inverted version of SCK.
5-8.

Troubleshooting the Keypad

To isolate a problem with the keypad, do the following:
1. Check the column (C0-C3) and row (R0-R3) lines that go to the keypad module.
With no keys pressed and the instrument powered up, the PWR_SW1 (P3-10) should
be at the battery voltage (5 to 6 volts). PWR_SW2 should be at digital ground.
Column lines C0 to C3 should be driven low. Row lines R0 to R3 should be pulled
high to VCC (3.3V). Thus, a key press should cause the appropriate row line to be
driven low momentarily. Refer to the keyboard schematic in Chapter 7 to identify
which row line will be driven low for a given key. If the column lines from the DSP
chip (U2) are not low while the instrument is running and updating the display, the
column line may be shorted to VCC or there may be a problem with the DSP chip. If
a key press does not drive a row line low, there is probably a problem with the
keypad or the elastomeric interconnect.
2. Check the IRQA* line while pressing a key. The IRQA line (U2-121 or U3-9) should
be asserted low when a key is pressed to cause a user interface interrupt in the DSP
chip (U2). If the IRQA line is asserted and the correct row line to the DSP chip is
also asserted but the key still is not recognized, there may be a problem with the
DSP chip.
The IRQA* line should be asserted by U3 if the RESET line (U3-2) is low and one
of the row line inputs to U3 is driven low by a key press. If the IRQA line is not
driven low under these conditions, U3 may be defective.
3. Verify that after the interrupt has occurred, the appropriate row line is driven low
when the appropriate column line is also driven low. This occurs during the interrupt
process when the microprocessor is decoding the keypad. See the keypad schematic
Troubleshooting
Troubleshooting the Digital Section
5
5-9