msp430f235

Akhilesh,

Thanks for your post and Happy New Year! I'm moving this to the MSP430 forum for you to get help with your questions.

Blake

1. It depends (always it depends..). If you use SPI or I2C it's generally quite easy to do without external crystal as the clock will be transmitted (synchronous) and thus the requirement for a exact frequency is not really there. You only have to stay between certain boundaries but they can be reached easily.

Things get more complicated for the asynchronous UART. There it depends on the clock frequency of the MSP and the baudrate you want to achieve. Most of the time your baudrate will not be an exact divider of the MSP core clock and so there will be some bit errors start to appear if the baudrate gets to high. You can get up to 115200 baud from 1 MHz calibrated DCO but it can already get messy there.

The internal DCO is quite voltage and temperature dependent and so it may work fine at room temperature but out in the field it may stop working because the frequency drifted too far.

2. Depending on the voltage of the GSM module - if it runs with up to 3.6V then you can power the MSP from the same source (LV-TTL) and you don't need level shifting. If it's really the 5V TTL then you will have to do some adaption. You may get it though with just some resistors.

To get a more definite answer can you tell us which GSM module you are trying to use? 

1. Sir is it necessary to take baud rate 115200.......Can I take baud rate 9600?

2. If I take baud rate 9600  then external crystal will be used or not?

3. I am using Quad band Gsm module SIM900 from SIM Com company. Which supports power supply 3.2v- 4.8v.

I checked the information I could find without registering with SIMCon (registering just to get datasheets? I don't think so - basically this brand just died for me).

It's not completely clear if the UART uses 2.8V or the supply voltage levels.

In both cases you should be fine, if you supply the GSM module with 3.2V up to 3.6V and  the module uses indeed the supply voltage, then you can interface it directly to the MSP.

If it uses 2.8V for the UART, you can supply the MSP with 2.8V and the GSM module with any voltage you like. Or you can still use 3.2V to 3.6V and apply some level shifting (or current limiting).

From what I found the module has some autobaud detection and is able to use most standard values up to 115200 (being standard), so you can use 9600 baud and for that using a crystal is not needed, the calibrated DCO values are working fine there. Though a more conservative approach would be to use a crystal (it's a tradeoff between cost and  frequency stability)