Appendix C: Keeping Time With an External Clock
Backup Battery
Note: Current firmware does not support sleep mode. An update will be issued to
address this issue. The content of this appendix may change depending on the
implementation.
The In-Radar traffic statistics generates time-stamped historical records that are saved in
the on-board FLASH memory. Hence, the radar needs to keep calendar date and time.
The time is initially set from an external controller, typically a PC running Houston
Radar configuration or stats analyzer software.
If the PD420 is used in a configuration that may disrupt power, an external clock backup
battery must be connected as suggested below.
Note: Alternatively the clock may be manually reset from an external controller to the
correct time once power is restored.
Step 1
: Set the “Enable Low Power Sleep” mode bit in the MD variable (this is factory
default so no operation is required if you have not changed it).
The radar will now enter low power sleep mode where it will maintain the clock once the
external VCC supply voltage enters the sleep region as shown in the provided diagram.
The radar will resume normal operation once the input VCC voltage returns to the “Run”
region.
Step 2
: Setup an arrangement to switch in a 4.5V to 6.8V clock backup battery into the
VCC line once main power is removed. This may be done two ways:
Method 1
: “Wire OR” the backup source and the main VCC through two low drop diodes
as shown on the next page. Diode voltage drop must be taken into account when
determining backup voltage. For example if the diode voltage drop is 0.6V the backup
supply voltage must be between 5.1 and 7.4 volt. We recommend a low drop (<0.3V) low
leakage diode particularly for the clock battery when using a lithium battery that cannot
tolerate any significant reverse charging current.
Method 2
: If the main supply will not be removed, but rather switched off, install a Zener
diode across the power switch such that the radar continues to receive between 4.5 and
6.8VDC when the power is turned off via the switch (with the rest dropped by the
zenner).
The power consumed by the radar in low power sleep mode is as follows:
Installed Configuration
Micro Amps
Consumed in sleep
Sleep Mode
Enter Voltage
Sleep Mode
Exit Voltage
Stand Alone PD420 Radar
165 uA
15
A
7V nominal.
6.5V min
8V nominal.
8.7V max
