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UPS Shield for Raspberry Pi

General Information

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Introduction

In this post you will find general information about the UPS Shield and the configuration that has to be done to the Raspberry Pi. 


UPS Shield 

The UPS Shield module is a supercapacitor based energy storage. It contains two independent DC/DC converters. The first one is a bidirectional step-up/step-down converter working as the interface between the stable 5V supply rail and the supercapacitor. During charging the converter works in step-down mode and transports energy from external power supply to the supercapacitor. In case of a power failure the load device (Raspberry Pi or another SBC) is supplied from the supercapacitor via DC/DC converter in step-up mode. The second DC/DC converter is a Front-end step-down converter. The only function is converting a high input down to 5.1V for the 5V rail. 


Technical Specifications

General

ParameterValueConditions
Max. Total Backup time
Ttb
Min. 57 s
Typ. 81 s
Operation until supercapacitors not discharged.
Tamb = 25 ºC. 
Backup time for Sort-term power failure
Tst
Min. 22 s
Typ. 31 s
Operation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Charge time (fast charge)
= default configuration
Typ. 25 sOperation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Operating time (slow charge) Typ. 60 sOperation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Operating temperature range (module)-40 ºC to +85 ºC
-25 ºC to +70ºC
Assembled PCB
Including Push-Button and Screw terminals
Operating temperature range (supercapacitors)-40 ºC to + 60ºCSupercapacitor type: 22F / 2.7V / DxL = 16 x 26 mm


Input

ParameterValueConditions
Input Voltage Range7V DC ... 28 DCat screw terminal
Average Input Current1 A
0.5 A
Charge mode+RPi3 Booting at Vin = 12 V
Charge mode+RPi3 Booting at Vin = 24 V
Current consumptionTyp. 15 mANormal operation (No charge mode, no Shutdown mode), Tamb = 25 ºC


Output

ParameterValueConditions
Output voltage range
4.75 V DC ... 5.25 V DC

Output current range 
0 Aavg ... 1 Aavg
Typ. output ripple
10 mVppAt shutdown mode, Tamb = 25 ºC, ripple at fSW = 1200 kHz


Supercapacitor Interface

ParameterValueConditions
Working voltage at supercapacitor contacts1 V ... 5.25 V



Peak current through supercapacitor7 A
Charging method7.5 W (fast charge, default) = 1.5 A at 5 V
2.5 W (slow charge) = 0.5 A at 5 V
Constant power charging
Constant power charging
Balancing+/- 300 mAActive balance circuit for two supercapacitors in series

  

The Real-Time Clock

The board contains a PCF8563 integrated circuit communicating via I2C-bus with the RaspberryPi. A Lithium Coin Battery supplies the RTC chip when the Raspberry Pi is not powered on. We recommend to use the CR2032 battery.


ParameterValueConditions
RTC chip typePCF8563Supported by the Raspbian OS
Battery holder CR20323 V, Lithium
Battery Lifetime> 20 yearsat capacity 200 mAh
CommunicationI2C-busPins 3 and 5 of the GPIO interface (SDA and SCL), requires RTC driver installed on the RPi


In the file /boot/config.txt uncomment the line


dtparam=i2c_arm=on


and at the end add the line


dtoverlay=i2c-rtc,pcf8563


The RS-485 interface


The UPS Shield module contains a 2-wire RS-485 (or RS-422) transceiver. The transceiver is driven by the Raspberry Pi UART interface on the GPIO14 and GPIO15 pins. We send and reveice data by /dev/ttyS0.


In the file /boot/config.txt add the line

enable_uart=1

ParameterValueConditions
RS-485 Interface Communication Type 2-wire, half duplexTransceiver chip ISL8483E
RX/TX direction controlAutomatic


Controlled by GPIO24
Diver enabled by a low TX data bit
Driver disabled 25 us after the end of a low TX data bit

Optional
Failsafe biasing390R to GND
390R to 5V

Line Termination120R390R//220R//390R+2*10R
ProtectionESD to +/- 15 kVNo Surge/Burst protection
Echo-featureRX always enabled
TX enabled at data
Optional
Data transmission speed 0...250kbps
slew rate limited
25 us transmission pause
between TX and RX required


The driver-enable line follows the data with a short delay (25 us) before disabling the driver. There is no need to adjust the time constant for different transmission speeds. The automation direction control works from zero to maximum transmission baudrate. The only requirement is a 25 us transmission pause between sent and receive data. Thi short time interval corresponds to a time interval of about 3 data bits at 115 kbps. At 9600 bps the 25 us time interval corresponds to a fraction of one data bit. 

Instead of keeping the transmitter enabled for the entire transmission, the driver is enabled on the leading edge of the start bit or any logic low at the driver's inout. It also disables the driver 25 us after the leading edge of the stop bit or any logic high at the driver's input. When the driver is disabed, the biasing resistors ensure the receiver's output is a logic high. 


Discharge supercapacitors


The on-board discharge load should be used before unplugging the module from the carrier PCB. It helps to avoid sparking and excessive current flow when discharging the supercapacitors. 


ParameterValueCondition
Discharge load2 Ohm
Meander PCB trace used as shunt monitor
at 25 ºC
Typical Discharge Time ar default 2x22F supercapacitors 1 min.Short the pads R83 with a tweezer 


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