Since this hardware will be used in many projects, we give you detailed description of schematics and PCB board realization.
We used Microchip dsPIC30f4013 because it is very powerful MCU with plenty of peripheral possibilities. If you need a lot of processing, such as fast ADC, this is right choice for you. Also, this MCU isn’t expensive, about 5 EUROs.
You can find detailed description and datasheet on Microchip site: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010345.
As you can see recommended basic shematics for MCU is used. There are oscillator, headers for pins, reset circuit, programmer header (PICKit 3 ready) and UARTs header for debugging purposes.
If you need schematic and PCB you can download it here.
If you inexperienced with modems, we recommend reading our articles:
- Introduction to GPRS modems
- AT commands GPRS connection tutorial using Siemens ES75 GPRS modem
- ATmega8 sending and receiving data via GPRS Siemens ES75 modem using AT commands
Before we start to explain part with Telit GM862 modem you should know that this modem is obsolete. But good thing is that knowledge from here is usable for other Telit’s modems (GE863, GE865, GL865 etc.), since AT commands set is same. Literature and datasheets for Telit’s modem are widely available at web. We prefer this location as very useful: http://www.roundsolutions.com/techdocs/
Let’s continue with schematic. Since we wanted to supply MCU with 5V and optimal power supply of modem is 3,7V, a level translator is needed. We used digital isolators TI ISO7221 as available component. We recommend TI TXS0108 bidirectional voltage-level translator for future usage. Resistors R4 and R5 adjust level to CMOS 2.8V that is used on modem. As level translators you can used transistor as we used Q1, Q2 and Q3 for ON/OFF, reset and power monitor lines.
Attention: Read carefully pin description of modem, especially for TXD and RXD. As you can read in datasheet pin 20 – C103/TXD is not TXD of modem, since explanation said: Serial data input (TXD) from DTE. So, this pin is actually RXD of modem and if you read these fast, errors are possible.
DC-DC power supply is made with LM2576-5 and MCP1827-ADJ/ET chips. First chip makes 5V for MCU and it can withstand input voltage up to 40V and switched work provides lower losses. Second chip makes 3.7V for modem. The relative currents peaks can be as high as about 2A so power supply must be designed in order to withstand these current peaks. Practical this means you should put high capacitance capacitors with low ESR (Equivalent Serial Resistance).
We planned to use this board for projects with dsPIC, and results will be placed here:
If you have any questions don’t hesitate to ask, directly or via our forum.