Real-Time Clock - Experimenting with Raspberry Pi - page 79

Performing a DMM Resistance Check
Use these steps for the DMM resistance check:

1. Attach one probe of your DMM (reading kΩ) to the +5 V line

of the PCB.

2. Attach the other probe to the SDA line and take the resistance

reading.

3. Reverse the leads if you suspect diode action.

On my PCB, I read 3 . 3 k Ω. Reversing the DMM leads should read the same (proving
only resistance). Performing the same test with the SCL input, I also read 3 . 3 k Ω.

Performing a Voltage Reading
Do not skip this particular test. The result of this test will tell you whether your Raspberry
Pi will be at risk.

1. Hook up your PCB to the +5 V supply it requires, but do not

attach the SDA/SCL lines to the Pi yet. Just leave them loose
for measuring with your DMM.

2. With the DMM negative probe grounded, measure the

voltage seen at the PCB's SDA and SCL inputs. If there is no
pull-up resistor involved, you should see a low reading of
approximately 0.07 V. The reading will be very near ground
potential.

On my unmodified PCB, these readings were +5 V because of the 3.3 k Ω pull-up
resistors. If this also applies to your PCB unit, a modification is required .

Performing a Tiny RTC Modification
If you have the exact same PCB that I used, you can simply remove resistors R 2 and R 3 (but
I would double-check with the preceding tests). These resistors are shown in Figure 4-3 .
Carefully apply a soldering iron to sweep them off the PCB. Make sure no solder is left,
shorting the remaining contacts. I highly recommend that you repeat the tests to make
sure you have corrected the pull-up problem and test for short circuits.