problem in receiving msg via gsm module and pic16f628a with .bas code

[paranz]

^ Clarification lang, which e-gizmo GSM module are you using? the old BENQ GSM module (not being sold anymore since last year) or the newer SIM340DZ-based GSM/GPRS module?

Also, FYI, the SIM340DZ-based GSM/GPRS module is now being replaced by a SIM9000 module as of last week. SIM340 and SIM900 are pin compatible anyways.

[jungardc2]

^ Clarification lang, which e-gizmo GSM module are you using? the old BENQ GSM module (not being sold anymore since last year) or the newer SIM340DZ-based GSM/GPRS module?

Also, FYI, the SIM340DZ-based GSM/GPRS module is now being replaced by a SIM9000 module as of last week. SIM340 and SIM900 are pin compatible anyways.

SIM340DZ-based GSM/GPRS module po..wala na kasi yun BENQ..

[paranz]

I see.. we are using the same GSM module

I have interfacing problem with this GSM module before when using PIC16F877A with VDD = +5v.

In my experiment, I used 3 different voltage translation circuits for testing.
1. Direct connect GSM_TX to MCU_RX and voltage divider circuit for MCU_TX to GSM_RX
2. Using NPN transistor fro GSM_TX to MCU_RX and signal diode for MCU_TX to GSM_RX
3. Using buffer chip

All configurations i tested with the PC + Hyperterminal and all work fine.

Now, when i tested to a PIC16F877A, (1) doesn't work reliably. I run the PIC + GSM module setup with a debugger (eICD2), and i can see 1 or 2 received characters that are wrong, i.e. instead of +CMTI="SM",1 in the buffer i get +CMTh = "SM"&1.

I tried (3) and it worked perfectly. I used a HC125 buffer chip and it worked. Pero hirap hanapin ang ganito na IC.

So I tried (2) also and it doesn't worked sometimes, with wrong characters received but not as often or as unreliable as (1). In my setup right now (2) worked just fine, but testing it on several GSM modules (10 GSM modules to be precise, during a seminar workshop last week in e-gizmo  )  it still not as reliable.

[jungardc2]

I see.. we are using the same GSM module

I have interfacing problem with this GSM module before when using PIC16F877A with VDD = +5v.

In my experiment, I used 3 different voltage translation circuits for testing.
1. Direct connect GSM_TX to MCU_RX and voltage divider circuit for MCU_TX to GSM_RX
2. Using NPN transistor fro GSM_TX to MCU_RX and signal diode for MCU_TX to GSM_RX
3. Using buffer chip

All configurations i tested with the PC + Hyperterminal and all work fine.

Now, when i tested to a PIC16F877A, (1) doesn't work reliably. I run the PIC + GSM module setup with a debugger (eICD2), and i can see 1 or 2 received characters that are wrong, i.e. instead of +CMTI="SM",1 in the buffer i get +CMTh = "SM"&1.

I tried (3) and it worked perfectly. I used a HC125 buffer chip and it worked. Pero hirap hanapin ang ganito na IC.

So I tried (2) also and it doesn't worked sometimes, with wrong characters received but not as often or as unreliable as (1). In my setup right now (2) worked just fine, but testing it on several GSM modules (10 GSM modules to be precise, during a seminar workshop last week in e-gizmo  )  it still not as reliable.

ahhhhhhh...salamat po sa experiment ninyo po..so this explains it..sa (2) switching npn lang po ba gawin ko? may 3.3v na zener kaya available? hehe try ko maghanap po pati yun hc125 (3)..

[0b00000111]

im trying this experiment po pero bakit po sa post na ito direct connected na si  GSM_TX sa MCU_RX:
http://www.electronicslab.ph/forum/index.php/topic,13813.0.html

and we all know that logic high level is  2.0v above and logic low level is from 0 to 0.8v..

ano ang voltage level ng GSM TX at idle?  sa PIC16F628A, based from block diagram ng RB1/RX pin, dumadaan ito sa schmitt trigger sa UART receive mode...
 
then based sa Input High Voltage parameter D040, pag schmitt trigger =  ang minimum na input high voltage is 0.8*Vdd. assuming 5V  ang gamit mo, 4V ang minimum input high voltage

[paranz]

then based sa Input High Voltage parameter D040, pag schmitt trigger =  ang minimum na input high voltage is 0.8*Vdd. assuming 5V  ang gamit mo, 4V ang minimum input high voltage

Ayun, dapat minimum of 4V ang Logic 1 papasok ng MCU_RX..

GSM_TX Vpk-pk is around 2.8V only, (Logic 0 = 0.1V, Logic = +2.9V) as shown in my scope.

For reliable UART reception by the PIC_RX pin, GSM_TX pin should transmit a logic 1 that is more than +4.0V. Kailangan talaga ng voltage level translator circuit to convert +2.9V to +5V.

[paranz]

ahhhhhhh...salamat po sa experiment ninyo po..so this explains it..sa (2) switching npn lang po ba gawin ko? may 3.3v na zener kaya available? hehe try ko maghanap po pati yun hc125 (3)..

Go for (3). I think meron LVC125 sa gizmo. For +3.3V you can always use a LM317 voltage regulator. But i typically use RT9163 voltage regulator from e-gizmo so that i can input +5v to it from the existing 7805 regulator on the PIC circuit and get an +3.3V output. With LM317 i need to use another DC regulator which is unwieldy for my setup, so RT9163 (or other LDO regulators) is better. Yun nga lang SOIC ito and not through-hole.

[paranz]

^ or use VDD = +3.3V instead of +5V. Problem solve

[0b00000111]

or pwede din ibaba na lang nya yung Vdd ng pic16f628a (but im not sure kung ano crystal value ni TS) para maabot yung 0.8*Vdd + some allowance...


or eto, ngayon ko lang naisip (cheap voltage level translator for this gsm modem).... resistor values are around 4.7k to 10k siguro hehehe



pag idle 2.8V sa GSM TX, OFF yung transistor, Vbe=0V, 5V ang papunta sa PIC
pag nag 0V ang GSM modem TX, may 0.7V sa Vbe, ON ang transistor, mag 0V papunta sa PIC....
(sorry kung puro ideal values hehehe)

unsimulated pa... pa comment na lang kung may mali or meron akong di naiconsider (tulad ng source/sink current nung TX ng gsm modem)... hehehe

[paranz]

^ yup that (2) circuit i'm recommending to TS. Medyo hirap lang kumuha ng stable na +2.8V

[0b00000111]

^^ pwede din revise na lang yung sa may base section for voltage divider for the ~2.8V (coming from the 5V regulator)...

[jungardc2]

^ yup that (2) circuit i'm recommending to TS. Medyo hirap lang kumuha ng stable na +2.8V

im trying hard to try this cricuit po..hirap nga mag2.8 v
anyway meron pala akong 74LS125..
meron na naman ako narereceive galing sa GSM but at 9600 baud rate i am always receiving only a limit of two characters and most of the time it is "at"..bakit ganun po?
i tried decreasing the baud rate and the character are different but bakit po 2 characters lang pa rin po narereceicve ko?

[0b00000111]

^^ kahit sa hyperterminal ganun rin?

[jungardc2]

^^ kahit sa hyperterminal ganun rin?

hindi po..

[Didoyskie]

ok lang ba to? direct connection from mcu to gsm...

pic16f628a(BOREN_OFF) yung supply galing sa VBat ng sim340DZ module.

nung tinest ko po... success sya... 

[jungardc2]

ok lang ba to? direct connection from mcu to gsm...

pic16f628a(BOREN_OFF) yung supply galing sa VBat ng sim340DZ module.

nung tinest ko po... success sya... 
you mean po isang psu lang gamit nyo?

[Didoyskie]

oo ung galing dun sa pin vbat ng sim340DZ...

around 3+V ata un eh...

gumagana nga sya ngayon eh... nakakareceive sya ng kahit anong word na iparead ko ("RING","OK")

tapos tama naman ung respond ng MCU.

[jungardc2]

yun bagong gsm ng e-gizmo po ba gamit ninyo?

[Didoyskie]

oo sim340DZ kakabili ko nga lang nito nung july... eto n b ung bago?

[jungardc2]

pareho po pala tayo..sige po salamat sa info..nakita ko na yun vbat. separate kasi yun psu q taps 2 characters lng limit ng nakukuha ko galing sa gsm papunta mcu..