Why does the TRF7960A EVM turn on with both EN and EN2 pulled low?

when the TRF79x0AEVMs is/are plugged in, running the as shipped firmware, it is running in a standalone polling mode first, looking for ISO15693, ISO14443A and ISO14443B transponders, and so then EN line should not be not be low. Pls. double check.

Hi Josh

The questions is not related to standalone mode or polling, but rather how and why is the TRF7970A booting.

On the EVM board the EN pin is connected to the MCU and the EN2 is connected to ground through a 10kOhm resistor. This means that both EN and EN2 will be low when power is applied to the board. According to the datasheet (page 18) this means that the TRF7970 is in total power down mode, and yet it is booting. But how and why?

In the datasheet (page 18 bottom - see text below), there is an explanation of the use of EN2. But on the EVM board the EN2 will never become high or have a rising edge, since the pin is connected to ground through the resistor as mentioned before.

"EN2 enables the start-up of the reader system from complete power down (EN = 0, EN2 = 0). In this case the EN input is being controlled by the MCU (or other system device) that is without supply voltage during complete power down (thus unable to control the EN input). A rising edge applied to the EN2 input (which has an approximately 1-V threshold level) starts the reader supply system and 13.56-MHz oscillator (identical to condition EN = 1)."

I hope you can shed some light on this issue.

Regards

Henrik

Hi

I’m a little concerned, that no one has answered the question, though it seems very simple and basic.  Does it mean that no one knows or can explain why the EVM board turns on???

I can see, that the same questions/issue has been raised before (a couple of years ago), with a similar result – no answer.

Regards Henrik

Henrik,

EN2 is tied to +5V in connector HDR_2, so VDD_X and SYS_CLK are fully operational.

Please take a look at Figure 2-4 in TRF7960EVM User's Guide.

Luiz

Thanks for your answer. If you are referring to figure 2-4 in the TRF7960 EVM User's Guide (slou192.pdf) it's just a picture of the top and bottom side of the PCB, it doesn't reveal the connection of the enable pins. If you instead look at the schematic for the EVM, you will see, that the EN2 is connected to pin 4 of  HDR_2 (as you also write), but in the schematic there is no connection to pin 4 of HDR_2 which means EN2 is left floating. However on the physical board pin 4 and pin 5 is connected through a 10 kOhm resistor (pin 5 is ground), hence EN2 is pulled to ground through a 10kOhm resistor.

Regards

Henrik

I asked you to look to Figure 2-4 and confirm the small jumper resistor soldered between pins 3 and 4 of HDR_2 (it's very small, but still recognizable). This is necessary only if you don't have one in hands.

I can't see this board working without this resistor pulling EN2 high. Please double check.

I have thoroughly checked again… and I think I have reached a conclusion.

On the EVM board there is a resistor that pulls EN2 to ground, which means that the EVM board should never come to life - but it does. Even if I remove the resistor and make short circuit to ground it comes to life. It is still a question why, but there might be a small clue in the datasheet on page 18 (SLOA743F), which say:

"A rising edge applied to the EN2 input (which has an approximately 1-V threshold level) starts the reader supply system and 13.56-MHz oscillator (identical to condition EN = 1)."

When power is supplied to the EVM board, it might briefly cause a rising edge, which is enough to activate the internal power supply, which once again activates the MCU, etc.. If this suspicion is right, the ability to turn on by the rising edge will partly rely on (lack of) decoupling of the supply and maybe also the grounding.

The conclusion is: in order to be sure, that the power supply of the TRF chip is turned on every time, the EN2 should be pulled high - this is also in accordance with the datasheet. And why the EVM board is turned on is still a question.

Fact: If EN2 is pulled to ground  VDD_X and SYS_CLK will turn off. This is exactly what happens with my board.

I hope you find an answer for the issue. Best regards!

Henrik -

just to re-iterate - you do realize that there is firmware involved here, correct? This is what is bringing EN line high which makes EN2 a "don't care"...then writing registers, etc. to bring the EVM into power up - so maybe i'm not clear on where you are exactly coming from and what your concerns are...let me know.

thanks!

Hi Josh

I'm aware the that the firmware is involved in the initialization of the TRF7960, however this I slightly more fundamental. If you look at the schematic of the TRF7960A-EVM, you will see, that EN2 is floating (not connected), but on the actual board there is a 10K resistor which pulls the EN2 to ground. With this in mind, how does the EVM board wake up, when EN2 is pulled to ground?

This problem started out with a board that I made based on the EVM board. My board would sometimes start, sometimes not, and it also depended on which power supply I used for the TRF7960. On the board I had pulled EN2 to ground, and unfortunately not through a resistor. I have solved the problem by cutting the connection to EN2, and solder a wire from the pin of QRF chip to VCC, thus pulling EN2 high. However it still remains an unanswered question why TRF7960 sometimes will wake up even with EN2 pulled low.

Regards

Henrik

Henrik -

I think you are confusing EN2 with EN...EN is tied to ground on the EVM through 10k, EN2 is tied to GND through 0 ohm jumper resistor (on HDR_2). if its not too much trouble - please confirm we are talking about EN or EN2.

Hi,

I'm also interested in the answer of this question and I think Henrik has been quite clear.

The firmware in the 430 should not be able to set EN _before_ the 430 is powered. According to the schematics I have the 430 is completely powered through the trf7970. On the TRF side EN2 is grounded by a 0-ohm resistor in HDR_2 and EN is pulled down through a 10k. At board power up Vin should powered but both EN and EN2 are grounded or pulled to ground. If the 430 is powered through vdd_x output how will be active if both EN and EN2 are gnd?

That EN2 is no care when EN is set is of course true, but that would require that 430 is powered before EN can be set. So just to be clear:

Is the 430 only powered through vdd_x from TRF7970? In my schematics it seems so.

How will the vdd_x become active if EN2 and EN is gnd (430 no yet booted due to vdd_x no yet active)?

BR

Hello,

In fact I admit I'm also confused. In my TRF7970AEVM EN2 is connected to ground through a small 0 ohm resistor and EN2 is pulled low through a 10K resistor.

According to the datasheet the MSP430 should not be started as it DVCC shouldn't have any applied voltage after start-up.

Is not important for my design as I will not feed my uC from TRF7970A but it is not consistent.

The input pin EN2 has two functions:

• A direct connection from EN2 to VIN to ensure the availability of the regulated supply VDD_X and an auxiliary clock signal (60 kHz, SYS_CLK) for an external MCU. This mode (EN = 0, EN2 = 1) is intended for systems in which the MCU is also being supplied by the reader supply regulator (VDD_X) and the MCU clock is supplied by the SYS_CLK output of the reader. This allows the MCU supply and clock to be available during sleep mode.

• EN2 enables the start-up of the reader system from complete power down (EN = 0, EN2 = 0). In this case the EN input is being controlled by the MCU (or other system device) that is without supply voltage during complete power down (thus unable to control the EN input). A rising edge applied to the EN2 input (which has an approximately 1-V threshold level) starts the reader supply system and 13.56 MHz oscillator (identical to condition EN = 1).

for further reading, please see section 5.2.3 here ==> http://www.ti.com/lit/gpn/trf7970a

- Josh

Thanks for the answer -but it still doesn't answer the very simple question: What makes the EVM board start-up?

Please look at the schmematic of the EVM board and the board itself. EN2 is connected to ground and same applies for EN until the MCU is turned on, which means that EN=0 and EN2 =0... With this condition, what makes the EVM board start-up?

/Henrik

Hello Josh,

I read several times these chapters in the datasheet and I'm not confusing EN with EN2. In my TRF7970AEVM EN2 is shunted to GND and EN is pulled low through 10K, factory configuration(schematic check, visual check and multimeter check). So this hardware sets EN=0 and EN2=0 on start-up, and that corresponds to Power Down Mode 1(Total PD). What makes VDD_X available in the start-up moment? EN=0 and EN2=0 should disable this. Or is another mechanism that enables VDD_X at startup? I think this is the question.

Best Regards,
Vitor Barbosa