This is preferable to a multi-key instruction such as:
1. Press key A, key B and key C simultaneously for three seconds, then release.
Multi-key instructions restrict the use of Atmel’s Adjacent Key Suppression™ (AKS™) feature,
available in many QT ICs, and may therefore require that key centers are more widely spaced.
3.2
Entry by External Command
Diagnostic mode can be initiated by external commands issued via RS-232, USB, or a similar
communications protocol. The interface can also be used to log data. However, the
disadvantages of this method are that:
• External equipment is required – which is not always convenient.
• Test results may be affected – for example by creating alternate paths to ground.
3.3
3.4
Entry by Option Setting
To initiate diagnostic mode, options such as jumpers, DIP switches, non-volatile memory
settings, etc. can be read at start-up and/or during operation. This method is particularly useful
for tests that cycle the product's power, but is not always convenient or easy to implement.
Mode Entry Considerations
Usually, it is advantageous to support all three methods of initiating diagnostic modes. If several
diagnostic modes are used, consider using a single entry method to step through the modes
(including normal display). The benefits are that:
• Operators need only remember a single key sequence or serial command.
• Code size is minimized.
• Additional diagnostic modes can be added without changing the base code or the product
documentation.
If the product has an OFF, CANCEL or CLEAR key, it can be used to cancel all diagnostics: for
example, by pressing the key for two or three seconds, or consecutively. This arrangement
offers users an intuitive exit from diagnostic mode.
Diagnostic software builds (i.e. using #ifdef DIAG) are also an option, but they are inconvenient
because the code is not available in the production compile. Often, optional code is not updated
with software changes, so it may not be usable when needed. Moreover, special compiles may
be necessary when product memory is limited.
4. Diagnostic Data Output
Diagnostic information can be read by using the product's own display and/or an external
interface. The product display (for example, an LED or LCD) is particularly convenient because
additional external equipment is not required.
An external interface such as RS-232 or USB allows data to be logged and commands to be
entered. The disadvantage is that external equipment is necessary, which may affect test results
(for example, by presenting alternate paths to ground).
External jigs with simple LED displays on PPK or binary interfaces are useful for products that
are not equipped with an MCU.
2
Diagnostic Modes for QT ICs
10704A–AT42–10/08
