Reply by Leon Heller February 3, 20062006-02-03
----- Original Message ----- 
From: "Ed Schlunder" <zilym@zily...>
To: <lpc2000@lpc2...>
Sent: Friday, February 03, 2006 7:41 AM
Subject: [lpc2000] Re: JTAG warning
	> --- In lpc2000@lpc2..., "leon_heller" <leon_heller@...>
wrote:
>>
>> --- In lpc2100@lpc2..., "leon_heller"
<leon_heller@h...> wrote:
>> > A client of mine who's been using the Nohau board for software
>> > development zapped the JTAG inputs to the LPC2106 by connecting
the
>> > adaptor while the board was powered up - something to watch out
for. I
>> > replaced the LPC2106 with an LPC2104 (all I've got here) and got
it
>> > working again.
>>
>>
>> Apparently, the JTAG was connected with no power to the board, which
>> seems strange.
>
> 1. Your first message says that they zapped the chip by:
>  a. Apply power to the LPC2106
>  b. Connect the JTAG interface to the LPC2106
>
> 2. The second message indicates that they actually zapped the chip by:
>  a. Cut power to the LPC2106
>  b. Connect the JTAG interface to the LPC2106
>
> Which case was it really? What JTAG interface circuit were they using?
> Were they using a wiggler clone?
>
> In case 1, I don't understand why this would zap the chip.
>
> Case 2 makes more sense to me. If the LPC2106 Vdd power supply is not
> powered, and you apply some 3V logic voltages to the JTAG pins, you're
> probably violating the chip's absolute maximum ratings. Most CMOS
> chips say input pins are not allowed to be driven higher than the
> power supply voltage, plus maybe a small extra amount (0.5VDC in the
> case of LPC2106). If Vdd is at 0VDC, then applying more than 0.5VDC to
> input pins will be exceeding the absolute maximum ratings.
>
> This is also a potential problem with the 74AC244 wiggler clones. If
> the 74AC244 is running off the target board's 3.3VDC, then we
> shouldn't drive the 74AC244 inputs beyond 3.3 + 0.5 = 3.8VDC according
> to the 74AC244 datasheet. However, the parallel port side of the
> wiggler clone is driven by 5V, doh! Maybe this could contribute to
> wiggler clones acting flakey?
>
> At least by powering the 74AC244 from the target's 3.3VDC, the wiggler
> clones should protect the LPC2106 from dying in Case 2 because the
> JTAG input pins will only be driven by the wiggler clone if the
> LPC2106 is already powered up (and thus powering the 74AC244).
>
> VHC logic chips are special in that they do not have the restriction
> that input pins can not exceed the power supply voltage level. That is
> why I think the parallel port JTAG debuggers should be using a
> 74VHC chip to convert from 5V logic to 3V logic. The 74VHC can be
> powered at 3V while still accepting 5V at it's inputs without over
> stressing the part.

That was a long time ago, and they weren't all that sure what they did. I 
can't even remember what JTAG interface they were using. I thought the 
parallel port uses TTL signals - VIH is 3.3 V max.

Leon
	

An Engineer's Guide to the LPC2100 Series