Interfacing 68hc908 using USB to TTL cable

Discussion in 'Embedded Systems and Microcontrollers' started by bthomas, Apr 27, 2013.

  1. bthomas

    Thread Starter Member

    Aug 15, 2009

    I am trying to program a 68hc908 using the bl08 bootloader, based on a circuit from and a USB to TTL cable.

    Unlike the circuit in the link above I have eliminated the MAX232 keeping the 1N5158 diode and 8.2v zener and using a 9 V supply with 7805 regulator. All voltages in my circuit are as expected in particular PTA0 is at 3.3v.

    I am using a Nokia CA-42 cable that has a PL2303 hacked as a USB to TTL (3.3v) cable. See for instance
    This cable does work and I can confirm that it passes a loopback test (connecting TX with RX) and the TX pin is at 3.3 V.

    However when I connect the cable TX to 68hc90 RX and vice versa (GND to power supply GND) bl08 gives an error "Serial port failed to receive a byte, read returned 0". Using a simple scope I can see that the PC is not able to change the voltages on the TX/RX pins. Is this because of low impedance of the MCU pins ?

    I would be grateful for any suggestions on what may be going wrong ? If it is indeed a problem with impedance then will a simple emitter follower transistor buffer solve the problem ? Do I need separate buffers for TX and RX pins ? Are there any tutorials you could point me to diagnosing and fixing such problems ? Please note I am trying to learn this MCU and its interfacing.

  2. Kees1948

    New Member

    Sep 6, 2018
    Hi Thomas,

    I am aware that it is old matter but it still may be useful for someone.
    The circuit for the bootloader contains an error I believe,
    The schottky diode should not have been in series with the TX input but in series
    with the RX output of the MAX232A.

    1N5818 |

    I expect that you passed over it long time ago :)

    best regards

  3. Kees1948

    New Member

    Sep 6, 2018
    My careful editing has been disrupted after posting, but the '+' should be connected together