UART connection to CMOS UART

Silver Diamond

I am working with Janus modem ( http://www.janus-rc.com/Documentation/terminus_cf_user_guide.pdf ) and trying to interface the TX and the RX lines. I am using eUSCI_A1 port which is on P2.5 (TXD) and P2.6 (RXD). When I step through port initialization as in the following code.

P2SEL1 = BIT0 + BIT1; /* Debug Port */
P2SEL1 |= BIT6; /* Modem Port */
P2SEL1 |= BIT5; /* Modem Port */

When last line is executed, error in console shows up:

MSP430: Can't Single Step Target Program: Could not single step device

At this point you cannot step forward. What can cause this? It is the output, why would this happen, do I need a resistor on the line?

over 13 years ago

0 Brian Boorman over 13 years ago

Guru 25210 points

Have you looked at either of these threads to see if they address your issue?

http://e2e.ti.com/support/microcontrollers/msp430/f/166/t/124140.aspx

http://e2e.ti.com/support/microcontrollers/msp430/f/166/t/18966.aspx

0 Silver Diamond over 13 years ago in reply to Brian Boorman

Genius 4010 points

Yes, I did look at these, I tried to slow the DCO rate down as well but that did not help. They funny thing is it is crashing only when I have the modem connected. I just do not understand why the setting bit 5 of P2SEL1 would cause this. This is the TX line. Is there something special about working with CMOS IO? The TX and RX lines are described on page 11 of http://www.janus-rc.com/Documentation/terminus_cf_user_guide.pdf. Section 9.1.

0 old_cow_yellow over 13 years ago

Guru 58965 points

The debugger itself is buggy.

0 Silver Diamond over 13 years ago in reply to old_cow_yellow

Genius 4010 points

But why would it happen only when the modem plugs in?

0 Jens-Michael Gross over 13 years ago in reply to Silver Diamond

Guru 227245 points

Silver Diamond said:
But why would it happen only when the modem plugs in?

Perhaps different GND levels? PC GND might be on a different potential than the modem GND, due to coupling in the switching power supply (usually not noticeable for humans, but with sometimes devastating efects on electronics).

unplug the modem and measure the voltage between USB GND and modem GND.

On HiFi systems, this is audible as the so-called 'ground-loop humming'.

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UART connection to CMOS UART