no pull down on LCD_RS

[jjkropp-Triton]

I have a question about the LCD_RS pin on the i.MX28. 

If the ENABLE_PIN_BOOT_CHECK is blown and LCD_RS is NOT pulled down, and is connected to an LCD(LCD_DOTCLK),  is the boot mode guaranteed to be determined by the OTP eFuse bits in ROM0 (HW_OCOTP_ROM0:0x8002C1A0:31:24)?  There is not a pull-up on LCD_RS either.

We have a module with a i.MX28 chip that is used in two different base-boards.  One base-board pulls down LCD_RS and the other does not.  Both base-boards use this pin for LCD_DOTCLK for it's display.  Both of these configurations are reading the OTP eFuse bits, which is the way we expect them both to operate.  I need to know if the base-board that doesn't pull down LCD_RS is guaranteed?  The reason we need this to operate without a pull down is for cross-base-board compatibility.  We have thousands of boards without a pull down on LCD_RS that originally did not need the ENABLE_PIN_BOOT_CHECK to be blown, but now require it.

Thanks,

JJ

[igorpadykov]

Hi Jed

it has not pull resistor, only keeper, pads with pull resistors are described in

Table 9-3. i.MX28 Functions with Pullup Resistors i.MX28 Reference Manual (rev.1, 9/2010)

http://cache.freescale.com/files/dsp/doc/ref_manual/MCIMX28RM.pdf

Best regards

igor

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[jjkropp-Triton]

Igor,

Thank you for your quick response.

My question in about an external pull-down resistor on LCD_RS.

In reference to section 12.2.1. and Table 12.9. ENABLE_PIN_BOOT_CHECK is blown.

We don’t have an external pull-down resistor (nor a pull-up) on LCD_RS.

Is it ok to NOT have a pull-down resistor on LCD_RS?

Is the boot mode determined(guaranteed) by the OTP eFUSE bits without it?

I’m asking this question because one of our designs has no pull down on LCD_RS and we need to read the boot mode from the eFUSE bits.

Thanks,

JJ

Section 12.2.1 reads as follows:

If LCD_RS is pulled down, then the boot mode is determined by the OTP eFuse bits, as

defined in Table 12-4.

[igorpadykov]

Hi Jed

according to i.MX28 design checklist

Hardware Design Checklist for i.MX28

If the ENABLE_PIN_BOOT_CHECK OTP bit is blown,

LCD_RS is high -> check boot mode on LCD_D[5:0]

LCD_RS is low -> check boot mode on OTP bits

~igor

[jjkropp-Triton]

Igor,

Is the Gate keeper sufficient for LCD_RS low without adding an external pull-down resistor?

Thanks,

JJ

[igorpadykov]

Hi Jed

keeper just prevents pad from floating, I believe

it is not reliable solution for such purpose.

~igor