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Interfacing LPC2148 UART with 3V CPLD

Started by Sutton Mehaffey October 10, 2007
I have a requirement for 3 RS232 links to my LPC2148. I have run one
directly to UART0. The others (for connecting a PC and a modem) I
have input into a 3V CPLD and am doing the switching in there to UART1.

However, I didn't take into account the signal differences. I was
using a Linear LT1382 between the PC and the CPLD, but that won't work
because the LT is a 5V TTL out and the CPLD can't handle that. I was
investigating options for this and found some 3V logic RS232 chips by
Linear (LT1342 and LT1330). Anybody else done this type of interface?
Any other vendors have compatible chips? I looked at Maxim and I
couldn't find anything. Thanks.

Sutton

An Engineer's Guide to the LPC2100 Series

Yes. But, I don't see that the logic levels are 3V. I think they are
5V which can't feed into my CPLD. Usually, there is a separate pin
for a Logic Supply (VL) if it can supply 3V logic.

Sutton

--- In l..., "rtstofer" wrote:
>
> --- In l..., "Sutton Mehaffey" wrote:
> >
> > I have a requirement for 3 RS232 links to my LPC2148. I have run one
> > directly to UART0. The others (for connecting a PC and a modem) I
> > have input into a 3V CPLD and am doing the switching in there to
UART1.
> >
> > However, I didn't take into account the signal differences. I was
> > using a Linear LT1382 between the PC and the CPLD, but that won't work
> > because the LT is a 5V TTL out and the CPLD can't handle that. I was
> > investigating options for this and found some 3V logic RS232 chips by
> > Linear (LT1342 and LT1330). Anybody else done this type of interface?
> > Any other vendors have compatible chips? I looked at Maxim and I
> > couldn't find anything. Thanks.
> >
> > Sutton
> > I use MAX3232 - it works at either 3.3V or 5V
>
> Richard
>
Thanks. I didn't catch that. That will solve my problem and be a
cheaper alternative than my current chip.

Sutton
--- In l..., "rtstofer" wrote:
>
> --- In l..., "Sutton Mehaffey" wrote:
> >
> > Yes. But, I don't see that the logic levels are 3V. I think they are
> > 5V which can't feed into my CPLD. Usually, there is a separate pin
> > for a Logic Supply (VL) if it can supply 3V logic.
> >
> > Sutton
>
> There is no separate connection for any voltage other than Vcc which
> can be ANYTHING from 3.0V to 5.5V with the usual choices being 3.3V
> and 5.0V.
>
> If the Vcc pin is at 3.3V then the chip can be directly connected to
> any 3.3V system.
>
> The ST3232 version of this chip is used by Digilent Inc on their
> Spartan 3 Starter Board with a Xilinx 3.3V FPGA although the schematic
> calls for a MAX3232. They do use 100 Ohm series resistors on the
> signals between the '3232 outputs and FPGA inputs, probably for
> damping purposes. There are no such resistors between the FPGA
> outputs and '3232 inputs. See page 7 here:
> http://digilentinc.com/Data/Products/S3BOARD/S3BOARD-sch.pdf
>
> Richard
>
--- In l..., "Sutton Mehaffey" wrote:
>
> Yes. But, I don't see that the logic levels are 3V. I think they are
> 5V which can't feed into my CPLD. Usually, there is a separate pin
> for a Logic Supply (VL) if it can supply 3V logic.
>
> Sutton

There is no separate connection for any voltage other than Vcc which
can be ANYTHING from 3.0V to 5.5V with the usual choices being 3.3V
and 5.0V.

If the Vcc pin is at 3.3V then the chip can be directly connected to
any 3.3V system.

The ST3232 version of this chip is used by Digilent Inc on their
Spartan 3 Starter Board with a Xilinx 3.3V FPGA although the schematic
calls for a MAX3232. They do use 100 Ohm series resistors on the
signals between the '3232 outputs and FPGA inputs, probably for
damping purposes. There are no such resistors between the FPGA
outputs and '3232 inputs. See page 7 here:
http://digilentinc.com/Data/Products/S3BOARD/S3BOARD-sch.pdf

Richard
--- In l..., "Sutton Mehaffey" wrote:
>
> I have a requirement for 3 RS232 links to my LPC2148. I have run one
> directly to UART0. The others (for connecting a PC and a modem) I
> have input into a 3V CPLD and am doing the switching in there to UART1.
>
> However, I didn't take into account the signal differences. I was
> using a Linear LT1382 between the PC and the CPLD, but that won't work
> because the LT is a 5V TTL out and the CPLD can't handle that. I was
> investigating options for this and found some 3V logic RS232 chips by
> Linear (LT1342 and LT1330). Anybody else done this type of interface?
> Any other vendors have compatible chips? I looked at Maxim and I
> couldn't find anything. Thanks.
>
> Sutton
>

I use MAX3232 - it works at either 3.3V or 5V

Richard