Lpc2000

----- Original Message ----- 
From: "Tom Walsh" <tom@...>
To: <lpc2000@yahoogroups.com>
Sent: Monday, January 16, 2006 9:58 PM
Subject: Re: [lpc2000] Building DIY wiggler w/74VHC08


> Ed Schlunder wrote:
>
>>I'm thinking of building the DIY wiggler so that I can program and
>>debug some LPC2103 chips on the cheap.
>>
>>I see the wiggler.txt schematic in the files area. It is using a
>>74AC244 chip to do voltage level conversion between the PC and the
>>LPC21xx chip. It only seems to use 4 line drivers of the 74AC244 chip
>>(half the chip is left unused).
>>
>>Would there be any problem with using a 74VHC08 quad 2 input AND gate
>>chip instead? I would just tie one side of each AND gate high so that
>>it can be used as 4 line drivers. This chip is cheaper and smaller
>>than the 74AC244.
>>
>>
>>
> However, it may not have the same drive levels (output current).  The
> '244 series is a buffer / driver, typically producting 12ma sink and 4ma
> source.  The '08 is a logic device, not a driver,  you will probably
> only find that it can do +/- 4ma (if you are lucky).
>
> Output impedance would dictate how quickly signals may be driven high /
> low on the JTAG interface.  Too high an impedance == lousy signal edges
> / ring / overshoot.

I think that is probably the case, contrary to what I said earlier. Ground 
bounce due to poor construction techniques may be one reason why some people 
have problems getting it to work reliably.

Leon