Lpc2000

--- In lpc2000@yahoogroups.com, "leon_heller" <leon_heller@...> wrote:
>
> --- In lpc2100@yahoogroups.com, "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.

--
K9spud Microcontroller Electronics
http://www.k9spud.com/