Register: Input Registers for 32-bit Ports A and B.

((PUB ┆ PRI))
   INA < [Pin(s)] >

((PUB ┆ PRI))
   INB < [Pin(s)] > (Reserved for future use)

Returns: Current state of I/O Pin(s) for Port A or B.

• Pin(s) is an optional expression, or a range-expression, that specifies the I/O pin, or pins, to access in Port A (0-31) or Port B (32-63). If given as a single expression, only the pin specified is accessed. If given as a range-expression (two expressions in a range format; x..y) the contiguous pins from the start to end expressions are accessed.

Explanation

INA and INB are two of six registers (DIRA, DIRB, INA, INB, OUTA and OUTB) that directly affect the I/O pins. The INA register contains the current states for each of the 32 I/O pins in Port A; bits 0 through 31 correspond to P0 through P31. The INB register contains the current states for each of the 32 I/O pins in Port B; bits 0 through 31 correspond to P32 through P63.

NOTE: INB is reserved for future use; the Propeller P8X32A does not include Port B I/O pins so only INA is discussed below.

INA is read-only and is not really implemented as a register but rather is just an address that, when accessed as a source item in an expression, reads the Port A I/O pins directly at that moment. In the result, a low (0) bit indicates the corresponding I/O pin senses ground, and a high (1) bit indicates the corresponding I/O pin senses VDD (3.3 volts). Since the Propeller is a CMOS device, the I/O pins sense anything above ½ VDD to be high, so a high means the pin senses approximately 1.65 volts or higher.

Each cog has access to all I/O pins at any given time. Essentially, all I/O pins are directly connected to each cog so that there is no hub-related mutually exclusive access involved. Each cog has its own INA pseudo-register that gives it the ability to read the I/O pins states (low or high) at any time. The actual I/O pins' values are read, regardless of their designated input or output direction.

Note because of the "wired-OR" nature of the I/O pins, no electrical contention between cogs is possible, yet they can all still access I/O pins simultaneously. It is up to the application developer to ensure that no two cogs cause logical contention on the same I/O pin during run time. Since all cogs share all I/O pins, a cog could use INA to read pins it is using as well as the pins that are in use by one or more other cogs.

Using INA

Read INA to get the state of I/O pins at that moment. The following example assumes Temp was created elsewhere.

Temp := INA                      'Get state of P0 through P31

This example reads the states of all 32 I/O pins of Port A into Temp.

Using the optional Pin(s) field, the cog can read one I/O pin (one bit) at a time. For example:

Temp := INA[16]                  'Get state of P16

The above line reads I/O pin 16 and stores its state (0 or 1) in the lowest bit of Temp; all other bits of Temp are cleared.

In Spin, the INA register supports a special form of expression, called a range-expression, which allows you to read a group of I/O pins at once, without reading others outside the specified range. To read multiple, contiguous I/O pins at once, use a range expression (like x..y) in the Pin(s) field.

Temp := INA[18..15]              'Get states of P18:P15

Here, the lowest four bits of Temp (3, 2, 1, and 0) are set to the states of I/O pins 18, 17, 16, and 15, respectively, and all other bits of Temp are cleared to 0.

IMPORTANT: The order of the values in a range-expression affects how it is used. For example, the following swaps the order of the range from the previous example.

Temp := INA[15..18]              'Get states of P15:P18

Here, Temp bits 3, 2, 1, and 0 are set to the states of I/O pins 15, 16, 17, and 18, respectively.

This is a powerful feature of range-expressions, but if care is not taken it can also cause strange, unintentional results.