ST7538 problem

[pmd]

Posted on February 03, 2006 at 09:57

ST7538 problem

[pmd]

Posted on February 01, 2006 at 11:37

Hi all ST7538 guru's.

I have a problem with a ST7538 that is programming itself. The problem is, that the first time I power it up, it somehow programs itself with some of the data on the TxD pin. The problem is, that I would like to use the default carrier frequency (5kHz), but it starts up transmitting with a frequency of 60kHz!! I have not tied the REG_DATA externally to GND, but what about the internal ''pull-down'' resistor? Isn't that good enough to tie it to GND, or do I need a external resistor? I have attached the circuit so you guys can see what I have made! I know that the datasheet recommends that I tie some of the other pins (like C_PLUS and C_MINUS), but that's not nessesary. I have made a build-up on protoboard, and this works great with this circuit. So the questions is, what can cause the control register to be programmed to something other than the default values? Could it be that the register could be programmed by acciddent by a power-up pulse or something like that? I'm about to rip my hair out... I have changed about 4 chips up till now, so I would like to know if anyone of you guys has been experienced something like I have! Regards. Peter ________________ Attachments : Schematic_til_ST7538_implementering_p__MK-II_printet_-_revideret_udgave.pdf : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006I095&d=%2Fa%2F0X0000000bUH%2FL8CmukUSu0zvBC5d67WdoIg14HjfRvlcXBOWcF0xYNc&asPdf=false

[pmd]

Posted on February 03, 2006 at 07:26

Hi again.

I have made a read out of the control register, and the control register looks fine. I is set to default, and nothing is wrong with the register. REG_DATA is low all the time, and the MCLK is 4MHz. BUT, still the transmitting frequency is about 60kHz!!!! What can cause the frequency to be different from the one set by control register?? Can I have made some layout faults that can cause this to happen? Regards. Peter

[pmd]

Posted on February 03, 2006 at 09:57

Hi again.

I would just inform you all that it now works. It seems that the fault was the supply. The supply on the PCB is a 5V. The 5V is then made to a 10V with a step-up converter, and this converter sometimes makes a power-up spike so the ST7538 chip won't start up correctly. I'm just glad to find the fault. Regards. Peter