PXAG49 does not echo inital character

[Rons]

hi,
    I have had good results with the P89C668, P89C51RD2BN, and
    P89C51RD2HBP. Lately I have tired the XA part and it does not
    appear to echo the initial character. The pinout of the P89C668
    and XAG49 are the same. The philips data sheet says to connect
    EA/WAIT(pin 35) to Vdd and leave ALE(pin 33)open. When I apply
    reset  I see the ALE pin pulse from high to low to high again. I see
    the clock out pin XTAL2 (pin 20) at 20MHz. But I think the device is
    locked up. There is one difference though, the data sheet says that
    a lower case "f" is to be sent. The three parts I have used before
    require a upper case "U". I also know about the EA/WAIT pin
    used for external access. But the data sheet says to tie it high
    for internal access only. I've been visiting this forum for a while
    and not much gets asked about the XA part. Any ideas.

[Andy Ayre]

The autobaud character is indeed a lowercase 'f'. Are you testing sending the 'f' using a terminal program? If you put a scope on RxD do you see the 'f' arriving? If so does anything appear to come out of the TxD pin?

[Rons]

Hi,
     I figured the problem out. The reset circuit has to be different than for
     the P89C668.  For the P89C668 I had a 10uf cap and push button
     switch in parallel , one side to 5V and the other side to RST (pin 10).
     I also had a 10K resistor and signal diode in parallel , one side to
     RST and the other side to ground.  This was a design I obtained from
     my work with the P89C51RD2BN.

     What works now is simply a 10k resistor from 5v to RST and a push
     button switch and 10uf cap in parallel , one side to RST and the other
     side to ground. The upper limit for baud rate is 19,200 using a 30MHz
     crystal.  I am actually using a flash programming tool I wrote using
     Tcl/Tk and C. It works as your FlashMagic program does but I needed
     something for Linux. You might want to push Philips to change their
     data sheets, they still recommend that people download WinISP.

     Thanks