Sim900 A6

friis

Senior Member
Hi,
I have 2 Sim900 A6 connected to Picaxe 28X2 (with an AXE 033 LCD attached) and I power the 28X2 from the A6:
VCC_IN on A6 to Picaxe Pin 20 (+V)
GND on A6 to Picaxe Pin 19 (-V)
U_TXD on A6 to Picaxe Pin 18 (hserin)
U_RXD on A6 to Picaxe Pin 17 (hserout)

The A6 is powered by a micro USB providing 5 V and 2 A.

That has worked for a while for both A6, but now I cannot talk to any of the A6 - I dont get a response to AT, etc.

I have tried to set RST on the A6 low for 100 ms, but that did not help (should it?).

Is my setup not allowed and have I destroyed the A6?. Any ideas?

Best regards
torben
 

friis

Senior Member
Hi,
I may add that I have never seen a green light on the A6 and the blue light only flickers on every now and then in spite of AT+CSQ readings of about 15,99. The A6 did, however, send and receive SMS OK.
torben
 

PieM

Senior Member
On A6 Shield Development Board, supply voltage is 4.8 - 9V . A MP1584 step down converter give 3.3V to the A6. (Avoid USB supply from a PC !)
But interface logic level is 3.3V ! So may be a problem on U-RXD ...
Test with a 4700uF capacitor to supply rail .
 
Last edited:

friis

Senior Member
Hi PieM,
My electronics knowledge is limited, but I assume that "on the rail" means placing the 4700uF capacitor between the TX pin on the computer and and the U_RXD pin on the A6. Does that ensure that the U_RXD pin is supplied with 3.3 v if the computer is supplied with approx. 5 v from the A6?
Do you think I have destroyed the A6?
Best regards
torben
 

PieM

Senior Member
"on the rail" means placing the 4700uF capacitor between Vcc and Gnd.
You need two resistors to get 3.3V from 5V Pin17 output.
And its better to use battery for GPRS module.

On a 28X2, HSerin & Hserout are Schmitt Trigger type. Status High if input >0.8 Vcc picaxe. See Input type on picaxe manual2
 
Last edited:

friis

Senior Member
Hi PieM,
I will get a 4700 uF capacitor tomorrow.
I assume "on the rail" refers to 28X2. It will then replace a 100nF capacitor I have there now.
torben
 

AllyCat

Senior Member
Hi,

It probably doesn't need to be as high as 4700 uF, but 100nF (alone) is probably far too small. However, don't Replace the 100nF, particularly with a value as high as 4700uF (which will have a significant series inductance), connect the electrolytic in parallel with the 100nF.

Cheers, Alan.
 

PieM

Senior Member
Hi,
Alan is right. But "I assume "on the rail" refers to 28X2. " is wrong . It is used to stabilize voltage and power flow of A6 (>2A).
 

friis

Senior Member
Hi PieM,
I have put the 4700 uF on one A6 and it made no difference:
They all (I have 3) act the same way: When I power them up the blue light flickers on irregularly and is heard from no more. And no green light. The one with 4700 uF on certainly is not on the net because when I call it, I am immeditaely informed that "the person on no... cannot answer the phone right now....". If I call the other A6, the phone rings. Actually I think they all 3 behaved like this right when I got them.
I have a Sim900 which goes on the net without problems.
Do you have any ideas as to what is wrong?
torben
 

friis

Senior Member
Hi PieM and AllyCat,
I am a bit confused. The capacitor goes on the A6. The capacitor should be smaller that 4700 uF - how much smaller? Or connect the 4700 uF in parallel with the 100 nF.
I will try that right away.
torben
 

friis

Senior Member
Hi PieM and AllyCat,
I have now connected:
- Mini usb supply power to the A6 (5 V, 2 A)
- VCC_IN and GND of the A6 are connected via 100 nF and 4700 uF in parallel
- pin 18 of 28X2 and U_TXD are connected
- pin 17 of 28X2 and U_RXD are connected
- the 28X2 is powered by a wall-wart

The blue light of the A6 does not light up and when I call the phone I am immediately informed that there is no connection.

If I connect the (other) GND pin of the A6 with GND of the 28X2 the red light cannot be turned comletely off - it keeps flashing. I thought such a connection was needed to create a common GND.

When I run the pgm on the 28X2 I get:

+CSQ: 15,99
+CREG: 1,13 (I cannot see an access technology code of 13)
+COPS: 2 (deregister from network)

If the connections are OK the A6 must be destroyed - right?
torben
 

PieM

Senior Member
100 nF is on the Picaxe. See manual...
GND pin of the A6 and GND of the 28X2 must be connected.
Always connect GND before VCC. Otherwise the module A6 can use the low voltage serial pins as ground and can get destroyed instantly.
 

friis

Senior Member
Hi PieM og AllyCat,
I have dropped the A6 for now. I am using SIM900. It can also be destroyed (I dont know how), but it seems more robust.
Thank you for your help.
Best regards
torben
 
Top