In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Hi Guys,

let me place some comments and answers to the requests I got via PM in the public:

1. Internal oscillator active with SX-Key connected

As long as the SX-Key is just connected to the OSC1/2 pins without actively generating an internal clock, this should not cause trouble at all. When you let the SX-Key generate a clock signal using the IDE's "run-Run", or "Run-Clock" options, an unpredictable situation might occur as the external clock fed into the OSC1 pin and the internally generated clock might interfere. I don't think that this will cause any danger to the SX-Key as its clock output line is protected against overloads. On the other hand, the SX might "go nuts" with such a combination - mabye "feeling" clocked at a high rate, so simply don't do it. The SX-Key and the SX-Key IDE can't take away all responsibility from the users, so "think before you click".

2.  External oscillator active with SX-Key connected

When a resonator or a crystal is connected to the OSC1/2 pins, or the output signal of an external clock generator is fed into the OSC1 pin, a similar situation arises when the SX-Key generates its own clock. Both signals are offending, and you usually will not be able to debug the device in this configuration. Most likely, the Debugger will display that the SX is running, and the debugger buttons are inactive. Again - simply don't do it - disconnect the external resonator, crystal or clock source before connecting the SX-Key for debugging. I never tested it but I assume that debugging won't be possible either if the internal clock generator is active.

On the other hand, when you just want to use the SX-Key to re-program the SX with an external resonator/crystal connected, this usually works out fine, i.e. the SX-Key takes over control by pulling up the OSC1 pin to 12V (the programming voltage Vpp). This makes the SX enter the programming mode, and it will no longer drive the resonator/crystal. After programming has finished, and Vpp has been removed, the SX usually does a reset, and starts executing the program found in program memory even with the SX-Key is still connected (not generating a clock signal, of course), driving the resonator/crystal to generate the system clock. Usually, crystals and resonators can stand the 12V Vpp, so there is no need to disconnect/remove them from pin OSC1 before programming the SX. This may be different with an externally connected clock generator. I once tried this, receiving a "Vpp generation failed" error from the IDE. This was because the external clock generator caused an excessive load to the programming voltage but neither the SX-Key nor the clock generator were damaged.

3. Parallel Resistors, Resonators/Crystals OSCHSx Settings

It never happened to me that an SX would not start generating the clock with no parallel resistor connected. Nevertheless, If you are going to design SX-based applications to be operated within the specified ambient temperature range (-40° ... 85°C), the parallel resistor would make sure that the clock starts running under extreme conditions. The same is true for the setting of the oscillator gain, controlled by the OSCHSx setting. To my experience, a 50 MHz ceramic resonator always worked fine with OSCHS2 and OSCHS3 where using OSCHS1 sometimes caused an unreliable oscillator startup.

4. Heat Problems

When you look at the diagrams presented in the SX28 datasheet, it shows a supply current of almost 90 mA at 5.5 V suply voltage at a clock frequency of 50 MHz when a resonator is used. This means that the SX consumes (i.e. dissipates into heat) almost 0.5 Watts at these ratings. Although not explicitely specified in the data sheet, I assume that this diagram does not consider any additional power dissipation caused by the currents and the voltage drops sinked and/or sourced by output pins. Therefore, it is normal that an SX gets warm, especially when operated at higher clock speeds, like 50 MHz. Nevertheless, in my applications I never found situations that the SX go so hot that you could not touch it with your finger. I had situations when this was the case but only when either outputs had a short, or when the SX was internally damaged.

Just to make it clear, I don't think that selecting a resistor parallel to the resonator, or the setting of the OSCHx option has a major impact on the dissipated heat.

5. Some precautions to protect your SX-Key and the SXes

Although the parts on the SX-Key board and the SXes are protected against electrostatic discharge (ESD) to a certain extend, it is a good idea to not go and test the limits. Therefore, don't connect/disconnect the SX-Key or the SX while the system is powered. Before touching components on your setup, discharge yourself by first touching a lead or surface that is connected to ground. If possible, avoid areas with floor carpets, or other materials that might cause charging your body when handling electronic components.

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