Lpc2000

I'm laying out a board to test the LPC2138 and looked at circuit 
diagrams for various 2106 and 2138 boards, to get the JTAG right.

Some of the diagrams (see Leon's in the file section) use a 74x244 to 
buffer the JTAG signals and optionally a transistor on the reset 
line. The Keil 2138 board only uses a few pull-up/pull-down 
resistors, no buffer/transistor (there are a few minor differences 
between 2106/2138 on activating JTAG).

On the IAR 2106 board I figured they did it just to isolate signals 
between the primary/secondary JTAG ports. 

I don't see any compelling reason to use the buffer unless you prefer 
to zap the buffer with static rather than the LPC. Or maybe if you 
have a questionable JTAG-parallel/usb converter.

Anybody (Leon?) have any insight?

thanks, Tom

--- In lpc2000@yahoogroups.com, "tkreyche" <tkreyche@w...> wrote:
> 
> I'm laying out a board to test the LPC2138 and looked at circuit 
> diagrams for various 2106 and 2138 boards, to get the JTAG right.
> 
> Some of the diagrams (see Leon's in the file section) use a 74x244 
to 
> buffer the JTAG signals and optionally a transistor on the reset 
> line. The Keil 2138 board only uses a few pull-up/pull-down 
> resistors, no buffer/transistor (there are a few minor differences 
> between 2106/2138 on activating JTAG).
> 
> On the IAR 2106 board I figured they did it just to isolate signals 
> between the primary/secondary JTAG ports. 
> 
> I don't see any compelling reason to use the buffer unless you 
prefer 
> to zap the buffer with static rather than the LPC. Or maybe if you 
> have a questionable JTAG-parallel/usb converter.
> 
> Anybody (Leon?) have any insight?

Signal integrity is important with interfaces like JTAG - clean sharp 
pulse edges are required for proper operation, although the 
frequencies are low. A buffer might not be essential in every case, 
but the cost is minimal, so it's a good idea to use one. 
Manufacturers' JTAG interfaces for other devices like FPGAs are 
always buffered, although I have heard of people getting away without 
them by keeping the leads short.

Leon