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Dake

Contributor

Registered: 2015-06-16

Posts: 32

ask modulation for PCF7931

Hello !

I would like to program functions to write on PCF7931 tags. The protocol uses an ASK modulation and I was wondering if functions already exists to config FPGA for the ASK ?

In proxmark3.h I found this configuration for FPGA in LF mode :

[== C++ ==]
// LF
#define FPGA_MAJOR_MODE_LF_ADC						(0<<5)
#define FPGA_MAJOR_MODE_LF_EDGE_DETECT				(1<<5)
#define FPGA_MAJOR_MODE_LF_PASSTHRU					(2<<5)
// Options for LF_ADC
#define FPGA_LF_ADC_READER_FIELD	 

Can MODE_LF_PASSTHRU  be useful for ASK modulation ?

Dake

Contributor

Registered: 2015-06-16

Posts: 32

Re: ask modulation for PCF7931

I wanted to say I saw the list of FPGA mods in the file armsrc/fpgaloader.h (not proxmark.h)
The ASK modulation that I would like to use has only 2 levels : a high and low amplitude at 125k. 
Do you think it is possible with the current code to do this or an improvement of the verilog code is needed ?

Dake

Contributor

Registered: 2015-06-16

Posts: 32

Re: ask modulation for PCF7931

I would like to generate this :

I searched if tag existing in the github already uses ASK modulation, I got the TI tag that uses an OOK modulation that looks like to the ASK. It uses this commands
:

[== C++ ==]
			// stop modulating antenna
			LOW(GPIO_SSC_DOUT);
			SpinDelayUs(1000);
			// modulate antenna
			HIGH(GPIO_SSC_DOUT);
			SpinDelayUs(1000);

Is it possible to control the voltage level when it's modulates antenna to do ASK ?

Thank you !

marshmellow

Contributor

From: US

Registered: 2013-06-10

Posts: 2,302

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Re: ask modulation for PCF7931

AFAIK, there is no tight control for voltages when modulating the antenna. you are modulating or not modulating.  (which is actually what you are showing in the image)

Dake

Contributor

Registered: 2015-06-16

Posts: 32

Re: ask modulation for PCF7931

Thank for your response Marshmellow ! You are right, I just saw that PCF7931 needs in fact a minimum modulation level between Vup and Vlow, so OOK (modulate or not modulate) is completely adapted.