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ST9 USER GUIDE

This style has the advantage of clarity. However, there is still a problem that it does not ad-

dress: the variety of the different products of the ST9 family.

The different products available in the ST9 family are very diverse and as a result it is not al-

ways the case for all products that a given function is performed by the same bit of the same

problem, the GNU9 programming tool chain provides a set of include files that define the phys-

ical location of every bit and register, by their symbolic name. These files correspond to the

appropriate variant of the ST9. Using them guarantees that switching to another member of

the family will not need more than changing the Include statement at the beginning of the

source text. The same example as above, using the predefined symbols reads as follows:

.include "system.inc" ; System register

MODER,#MOm_sspm+MOm_uspm+MOm_div2m; Both stacks in

;registers, clock divided by 2

spp

#SPI_PG

;SPI page

SPICR,#SPm_spen+SPm_SP_16

; Enable SPI,1st clock

; configuration, Clock/16

This notation is more compact, and is independent of changes either between variants in the

same family, or by changes made globally to the family in the future. This writing style is rec-

ommended for this reason.

4.2 PARALLEL I/O

The parallel input-outputs have a basically very straightforward functionality. Once initialized,

they appear as a register that can be written or read. However in many cases, direct byte-wide

input-output is not sufficient. Bit-oriented I/O is often what is used in microcontroller systems.

A powerful feature of the ST9 is that you can address the eight bits of each port individually.

The ST9, like most microcontrollers also provides the external pins of the other peripherals

(timers, UARTs, etc.) by diverting some bits from the parallel I/O ports.

The ST9 parallel I/O has an additional very flexible feature. You can independently configure

each bit as:

– An input with two variants (TTL or CMOS levels)

– An output with also two variants (open-drain or push-pull)

– A bi-directional port with either a weak pull-up or an open-drain output side

– An alternate function output (that is, the output pin of an internal peripheral), with also either

open-drain or push-pull output driver.

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