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Development tests of DIY GPS for Nikon DSLR

Test equipment

  1. Siglent SDS1204X-E oscilloscope
  2. GOPHET CPS-3205 II power supply
  3. UNI-T UT61E DMM
  4. Siglent SDG1032X signal generator

Tested Circuit

DUT

Tests without the GPS module

The GPS module is hard to manually solder and especially desolder. Thus, before assembling it, I wanted to make sure that the circuit works as expected.

Power-on

Power-on without GPS

Channel Signal
CH1: 5V
CH2: 3.3V_EN (U4.3)
CH3: 3.3V
CH4: GPS_OUT

Power-off

Power-on without GPS

Channel Signal
CH1: 5V
CH2: 3.3V_EN (U4.3)
CH3: 3.3V
CH4: GPS_OUT

On/off button

Turning 3.3V on

Power-on without GPS

Channel Signal
CH2: ON_SW (U3.1)
CH3: 3.3V_EN (U4.3)
CH4: 3.3V

Turning 3.3V off

Power-on without GPS

Channel Signal
CH2: ON_SW (U3.1)
CH3: 3.3V_EN (U4.3)
CH4: 3.3V

Logic level shifting

Logic level shifting was tested by injecting pulses to the input of U2. Pulse frequency was 2400Hz and amplitude 3.3V.

Rise time

Power-on without GPS

Fall time

Power-on without GPS

Output pulses with 3.3V input

Power-on without GPS

Logic level shifting with 2.475V input

According to ORG1411 datasheet, it has the following logic levels:

Parameter Symbol Min Max Unit
Output Voltage Low State VOL 0.4 V
Output Voltage High State VOH 0.75*Vcc V

Vcc in this circuit is 3.3V, therefore, VHIGH is 2.475V.

Later, tests with GPS module demonstrated that these pulses don't reflect the real-world voltage levels.

Power-on without GPS

Channel Signal
CH1: GPS_TX (C2)
CH4: GPS_OUT

Tests with the GPS module

The same tests were performed with the GPS module assembled.

Power-on

Power-on without GPS

Channel Signal
CH1: 5V
CH2: 3.3V_EN (U4.3)
CH3: 3.3V
CH4: GPS_OUT

Power-off

The pulses on the GPS_OUT line are UART transmissions of the GPS module.

Power-on without GPS

Power-on without GPS

Channel Signal
CH1: 5V
CH2: 3.3V_EN (U4.3)
CH3: 3.3V
CH4: GPS_OUT

On/off button

Turning 3.3V on

Power-on without GPS

Channel Signal
CH2: ON_SW (U3.1)
CH3: 3.3V_EN (U4.3)
CH4: 3.3V

Turning 3.3V off

Power-on without GPS

Channel Signal
CH2: ON_SW (U3.1)
CH3: 3.3V_EN (U4.3)
CH4: 3.3V

Logic level shifting

Rise time

Power-on without GPS

Fall time

Power-on without GPS

Output pulses with 3.3V input

Power-on without GPS

Channel Signal
CH1: GPS_TX (C2)
CH4: GPS_OUT

This test shows that the real logic levels of the ORG1410-PM04 UART is 1.8V. According to the SN74LV1T126DBVR datasheet, it is not supposed to work with 1.8V input when powered with 5.0V. This test shows that it works well, however, its performance can't be guaranteed.

UART waveforms

Power-on without GPS

Power-on without GPS

Channel Signal
CH1: GPS_TX (C2)
CH4: GPS_OUT
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