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263 With all of the mode values in the table above, the analog comparator’s positive input is AIN0 (Port B, bit 2) and the comparator’s negative input is either AIN1 (Port B, bit 3) or, if the ACME bit is set in a CPU register (see below), the analog input specified by the multiplexor select bits in Register.ADMUX. On the ZX-24 models, AIN0 is common with Port A, bit 2 so the latter I/O pin will need to be configured to be an input in high-impedance mode. Also, on the ZX-24 models built using boards earlier than Rev 5 (see the bottom side of the board), AIN1 has no external connection so the negative input must be supplied via the analog multiplexor. The ACME bit is in Register.SFIOR for the mega32 and mega128-based ZX models and in Register.ADCSRB in the mega644, mega644P, mega1280 and mega1281-based ZX models. Another option for the positive comparator input is to select the internal “band gap” voltage. This voltage level (approximately 1.23 volts) is selected by adding &H40 to the mode values in the table above. The built-in constant zxAnalogReference has this value. See the section in the Atmel microcontroller documentation describing the analog comparator for further details. Pin Change Interrupts For ZX models based on the mega644, mega644P, mega1280 and mega1281 CPUs, a task may await a state change on one or more pins of an I/O port. This mode is selected by specifying a special value for the intNum parameter according to the tables below for the respective CPU types. intNum Values for Pin Change Interrupts Value Built-in Constant Trigger – mega644,P Trigger – mega1281 Trigger – mega1280 &H20 WaitPinChangeA Pin change on Port A &H21 WaitPinChangeB Pin change on Port B Pin change on Port B Pin change on Port B &H22 WaitPinChangeC Pin change on Port C &H23 WaitPinChangeD Pin change on Port D &H24 WaitPinChangeE Change on Port Bit E.0¹ &H29 WaitPinChangeJ Pin change on Port J² &H2a WaitPinChangeK Pin change on Port K ¹Not available on the ZX-1281e. ²Bits 0-6 only. For each of the intNum values in the table above, the mode parameter specifies pin change interrupt enable bits corresponding to each pin of the port. For example, if the mode value is &H21, a pin change interrupt will be generated if either bit 0 or bit 5 of the specified port changes state. Clearly, a mode value of zero is useless since no pin change interrupt can ever occur in that case. When the trigger condition occurs an interrupt will be generated and the task awaiting the interrupt will rise to the highest priority. This will cause an immediate task switch meaning that the next instruction that executes will be the one following the WaitForInterrupt() invocation. Note that if another task performs an action that causes interrupts to be disabled, response to the interrupt will be delayed until interrupts are re-enabled. The fact that the current task is locked does not prevent the interrupt task from executing next. Interrupt Priority If two or more interrupts occur simultaneously, the task awaiting the highest priority interrupt is activated first. The priorities of the various interrupts are given in the table below.