AGND: Ground for use by analogue circuits. This is internally connected to GND at a single point close to the SPE's microcontroller. In order to minimize noise pick up this should be kept isolated from GND within external circuits.
AVCC: Positive 3.3V power supply for use by external analogue circuits. This is internally connected to VCC at a single point close to the SPE's microcontroller. In order to minimize noise pick up this should be isolated from VCC within external circuits.
GND: Ground for use by digital circuits. See also AGND.
VCC: Positive 3.3V power supply for use by digital circuits. See also AVCC.
VIN: This is the positive supply input to the SPE. Since the SPE can take an input of 5-36V the voltage on this pin can also cover this range. This pin is more likely to be useful when the input voltage is known.
VREF+: This is the ADC reference voltage output. The voltage is nominally 2.5V with a maximum error of 4%.
VeREF+: This pin is the input for an external reference voltage for use by the ADC.
AIN1 & AIN2: These are the two analogue inputs. Operation input range is 0V to VREF (+2.5V).
XIN1 & XIN2: These are the two digital inputs. Each input has a 10k resistor into the base of a BC847 NPN transistor. Minimum high input voltage is approximately 700mV. Maximum input voltage is 50V.
XOUT1 & XOUT2: These two outputs are open drain 2SK2158 N-channel MOSFETs. These are rated at 50V and 100mA but in practice the drain current should be kept well below this or order to avoid their power dissipation limit of 200mW.
IO1: This I/O line is directly connected to the microcontroller. It currently has no assigned function or software support.

Top of page

Power Supply

The Harvest SPE is designed to be powered from a nominal 12V DC supply but can be operated from a supply voltage of between 6 and 30V DC. Note that a 24V automotive supply is around 28V when under charge and may have transients exceeding the 30V limit. Because of this Harvest does not recommended that such a supply be used without additional over voltage protection.

Caution: The Harvest SPE must only be powered from an approved SELV power supply.

Due to the transmit bursts the ratio of peak to average current for any GSM phone is quite high. In testing we found the highest instantaneous peak current the SPE drew from a 12V supply was around 600mA. Following are some indicative average current consumptions from a 12V supply for various operating modes.

Operating (polling inputs and logging) in low power mode with GPRS modem disconnected and no terminal device connected to the RS232 port - 300 microamps
Operating with GPRS modem connected but not sending data (most common connected state as data bursts are brief) - g24 value needs to be measured

Note that sleep mode is enabled by setting the S24 register to a non-zero value (e.g. ATS24=5). The difference in current with and without the terminal connected is due to the current in the RS-232 data lines. Because the SPE uses a switch mode voltage regulator the power remains roughly constant as the supply voltage changes. This means that doubling the supply voltage will roughly halve the supply current.

Top of page

LED Indicators

The SPE has two LED indicators: one red and the other green. Both LEDs turn on when power is applied and stay on until the DSR signal from the GPRS module becomes active. This takes approximately five seconds and provides a useful indication of power up. The red, error, LED will turn on in the event of an error and stay on until a good connection is established. An error response to a modem command or a PPP negotiation failure will turn the red LED on. The green, carrier, LED corresponds to the carrier detect signal and is turned on while a good connection exists.

Top of page