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ZiLOG publish a circuit for "Interfacing the On-Chip Debugger’s DBG Pin with an RS-232 Interface" in (at least) the following places:
The
circuit is reproduced at right under the "Fair Use" clause of the
Copyright Acts.
Tiny-TIMs shipped before July 17, 2004 used this circuit exactly as published. The RS-232 Transceiver was an ST3232CD with 0.1µF charge pump capacitors and a 0.47µF supply decoupling capacitor (not shown in ZiLOG's schematic) and the diode was a 1N4148.
It has been discovered that there is a flaw in ZiLOG's design. Under worst-case conditions, the circuit may not pull the DBG pin of the MCU down to the required logic low level of 0.2*VDD. These conditions occur for low VDD, a low required logic zero voltage at the DBG pin, high logic zero output of the transceiver and high diode voltage drop.
No reports have been received of any problems whatsoever with ECROS Technology Tiny-TIMs. Therefore, it is possible that these worst-case conditions rarely, if ever, occur. Nevertheless, in Tiny-TIMs shipped after July 17, 2004, the 1N4148 diode has been replaced by a BAT42 schottky diode. This guarantees correct operation even under extreme worst case conditions.
If, for any reason, you want to update your Tiny-TIM with the schottky diode, ECROS Technology will send you one diode for each Tiny-TIM you have purchased at no charge, post-paid. There is no need to remove the 1N4148 diode, the BAT42 can be soldered in parallel with it, anode-to-anode and cathode-to-cathode. Your satisfaction is important. Please contact ECROS Technology if you are not happy with the above solution to the flaw in ZiLOG's published design for the TIM.
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