[WP34s] Help Flashing!

[matthiaspaul]

(11-03-2015 05:06 PM)emece67 Wrote:  I urge you to provide good connections for the serial interface; use fresh batteries (or supply the machine from the USB-serial converter); insert small (say, 100 Ohm - 1 kOhm) resistors in series with the RX/TX lines;

I would like to second this. Both a proper and stable power supply and the series resistors are quite important for various reasons. One of them is to limit the potential current through the clamping diodes in the chip in case the voltage on the RX input pin would become higher than the microcontroller's supply voltage (or lower than 0V). The clamping diodes exist to protect the internal circuitry, but if they get damaged they will make the corresponding port pin unusable. This scenario is quite likely to happen if the batteries aren't full any more or if the calculator has been left without batteries for a while (so that the capacitors between supply and ground have discharged). In this case, the microcontroller not only gets reverse powered through the input (which may already exceed the current limit of the affected clamping diode), but if the batteries are empty or the capacitors discharged, it is basically like driving the serial input signal against a short circuit for a moment - definitely too much for the clamping diodes. A firm low-resistance connection between the I/O power supply that powers the source of the serial signal (f.e. the 3.3V output of a FTDI chip) and the supply plane of the calculator as well as a similarly good ground connection will prevent this for as long as these connections are established before the serial line is driven high. However, if this connection fails for some reason, it may likely damage the chip when logical high level is asserted on the serial line, at least when the series
resistor(s) aren't present.

The drawback of the series resistors is that they will limit the allowable length of the serial data line.

BTW. A low-resistance connection to the external power supply typically means that the calculator's batteries would be connected almost directly to the external supply. Since this may damage the batteries if the external supply voltage is higher (or lower), it is a good idea to remove the batteries from the calculator for as long as the calculator is powered externally or to add circuitry to decouple the battery supply path from the external supply path through a MOSFET switch (or - significantly less efficient - through an ultra-low drop Schottky diode).

Greetings,

Matthias