Question about JTAG pins as GPIO on LPC2148

Started by senhoruilson February 4, 2009
Hello All, im designing a weather datalogger using LPC2148 and i ran
out of available GPIO pins to accomplish all the functions i need (
unless i use the JTAG pins as GPIO ).

So, here is my idea on how to use the share the JTAG pins without
loosing the JTAG functionality completelly (i would share the JTAG
pins with my LCD control GPIO pins .)

I could have a jumper on a GPIO pin.

IF the jumper is set as HIGH, the LPC2148 would startup normally ( no
JTAG, LCD instead ) .

ELSE , the LPC2148 would startup on the "jtag/debug/trace" mode ( no
LCD )
Was i clear ? Do you think it is going to work ? Do you have any
better idea on how to deal with the problem ?

Thanks for the attention.
My best regards,
Lucas Santiago

An Engineer's Guide to the LPC2100 Series

Lucas,

You have many options of extra I/O, for example an I2C port expander
available from NXP, Microchip.

Jan

--- In l..., "senhoruilson" wrote:
>
> Hello All, im designing a weather datalogger using LPC2148 and i
ran
> out of available GPIO pins to accomplish all the functions i need (
> unless i use the JTAG pins as GPIO ).
>
> So, here is my idea on how to use the share the JTAG pins without
> loosing the JTAG functionality completelly (i would share the JTAG
> pins with my LCD control GPIO pins .)
>
> I could have a jumper on a GPIO pin.
>
> IF the jumper is set as HIGH, the LPC2148 would startup normally (
no
> JTAG, LCD instead ) .
>
> ELSE , the LPC2148 would startup on the "jtag/debug/trace" mode (
no
> LCD )
> Was i clear ? Do you think it is going to work ? Do you have any
> better idea on how to deal with the problem ?
>
> Thanks for the attention.
> My best regards,
> Lucas Santiago
>

senhoruilson ha scritto:
>
> Hello All, im designing a weather datalogger using LPC2148 and i ran
> out of available GPIO pins to accomplish all the functions i need (
> unless i use the JTAG pins as GPIO ).
>
> So, here is my idea on how to use the share the JTAG pins without
> loosing the JTAG functionality completelly (i would share the JTAG
> pins with my LCD control GPIO pins .)
>
> I could have a jumper on a GPIO pin.
>
> IF the jumper is set as HIGH, the LPC2148 would startup normally ( no
> JTAG, LCD instead ) .
>
> ELSE , the LPC2148 would startup on the "jtag/debug/trace" mode ( no
> LCD )
>
> Was i clear ? Do you think it is going to work ? Do you have any
> better idea on how to deal with the problem ?
>
> Thanks for the attention.
>
> My best regards, Lucas Santiago

1. I would discourage you to use jtag pins; probably you will not use
trace so you can use those pins.
2. If you need more pins you may try to modify your hw in a different
way or you may add an i2c or spi peripheral or just a s/r. If you have
an lcd probably you have also a keyboard; a good idea could be to reduce
the keyboard pin count using some skilled solution or multiplexing
keyboard and lcd pins.
>
>



Tnx for the help.

Manca : The trace pins are already being used by the graphic LCD as an
8 bit data bus. I need the JTAG ones now , unless i add a port
expander to my design.

Why would you not recommend using the JTAG pins ? is it a matter of
loosing the debug functionality or is it something else that i cant
see ? I would really preffer this solution than the port expansion
one.
About the i2c port expansion : how fast should i expect the pin
reaction delay to be ?

thanks a lot !
Lucas

senhoruilson ha scritto:
>
> Tnx for the help.
>
> Manca : The trace pins are already being used by the graphic LCD as
> an 8 bit data bus. I need the JTAG ones now , unless i add a port
> expander to my design.
>
> Why would you not recommend using the JTAG pins ? is it a matter of
> loosing the debug functionality or is it something else that i cant
> see ?
1. yes, you may need to debug also when your board will be in mass
production
2. because those pins goes in a predefined state during initial reset
and they are correctly managed only after the reset as you program
internal mcu registers.
I would really preffer this solution than the port expansion
> one.
>
> About the i2c port expansion : how fast should i expect the pin
> reaction delay to be ?
I already used for normal i/o and also for text or graphics lcd and I
really have no troubles. You may find 400kHz i/o port expansion also 8
or 16 bit wide or also more. You may also find lcd controllers (also 7
segment lcd controllers) I used a Philips one and all went ok.
>
> thanks a lot ! Lucas
>
>