Most semiconductor manufacturers seem particular about using their own products when possible. For example, the Analog Devices Blackfin development board uses another Blackfin processor for the JTAG adapter, as well as numerous AD components in the analog signal chain (see board photo here). This seems true of TI's development products as well, for example the Beagleboard, which was developed with TI support, seems to use a fair amount of "in-house" components (TI's DVI transmitter as well as their power management chip) original Beagleboard.
Therefore, it seems odd that a company that makes a large range of microcontrollers and CPUs ranging from the 16-bit MSP430 to high end ARM-based OMAP processors would completely avoid their own designs in their line of popular graphing calculators. Their older designs seem to use Z80 and 68000 CPUs, while their more recent stuff uses processors from LSI logic disassembled photos here.
I have to imagine that the latest versions of their CAS (Computer Algebra System) software is mostly written in portable code – and they had no problem porting it to the relatively obscure LSI ZEVIO architecture for their nSpire series (which is based on ARM9 + proprietary DSP), so it does not seem that the limitation is technical. Are TI products too readily available, so they are afraid of cheap Chinese clones appearing with pirated firmware? Are the reasons purely economic? — I can't imagine the couple of cents in parts per unit difference being a big issue since the margins are probably in their built-in CAS software. Are there some weird politics at play that we don't know about?
Best Answer
You are confusing development boards which are really eval boards which demonstrate that vendors product. And a completely separate division of the corporation where the guts are not normally visible to the customers, and price and battery life, etc are a major factor. You will get the occasional CEO that dictates for a year or so or a product or two that we must use our sister divisions products, often destroying a product line or two, sometimes killing a division, but unless there is such a mandate the divisions likely were at one time completely separate companies acquired at different times and other than the logo on the front of the building have absolutely nothing to do with each other, and unless forced to do not communicate with each other in any way.
When the chip folks make an eval board it makes a lot of sense for them to use their own parts, its a lot cheaper, they know their parts, it is all in house, and there are no battery or other requirements, cost if anything and using their own is often cheaper so even if more complicated or not the right solution they will use another one of their parts.