Other Parts Discussed in Thread: ADS7843, ADS7846, TSC2046, TSC2008
Dear Technical Support,
We have an established Point of Sales product line with a 4-wire resistive touch screen interface based on the SEMTEC SX8650 I2C interfaced TSC chip (12DFN package). This chip has recently been re-categorized as obsolete with no alternative from the manufacturer.
This is requiring us to modify the design to incorporate an alternative. After some research on my part it appears that TI has a line of parts that I hope will suit our application and provide a stable multiple-part alternative source into the future. I am considering the TSC2046EIRGV as the target part to use for the redesign.
Below is a rationale that I have developed for this component selection. I am hoping you will be willing to review it and indicate if...
...my understanding about the product relationships is sound.
...this is a good strategy to ensure a long lived reliable supply with useful multiple alternative drop-in part choices.
... all or some of the mentioned parts are recommended for new designs.
Also would you recommend anything other than this product choice?
Thanks in advance for your help.
Rationale:
We do already have a SPI interface established in the design and prefer to use that interface over the I2C we used with the legacy part.
Our manufacturing process is not conducive to using BGA packages and an SSOP sized package would present more issues with fitting it into the existing PCB layout, so it would seem that the VQFN-16 package would be the easiest to implement.
There appears to be a design progression with this part...
ADS7843---ADS7846---TSC2046I---TSC2046E---TSC2008I
The ADS7843 is only available in an SSOP package. The four parts beyond the ADS7843 are all available in a VQFN-16 package. This package would be preferred over a (T)SSOP (physically too big) or a BGA (assembly and inspection difficulties).
These parts all have a SPI interface and have drop-in compatible footprints in the VQFN-16 package.
All of these VQFN-16 parts are listed as ACTIVE by TI and I have found various levels of distributor stock available for all of them.
I am thinking the fundamental “command based” TSC functions are carried through intact and associate with the same pinout across the three parts of the ADS7846 to TSC2046E range with various ancillary features added and some associated with extra pins of the package. We do not need any of the added features beyond those available on the ADS7846. So I am thinking the TSC2046 driver in the library of our firmware development platform (as we would use it) may extend across this 3 part number range since we would not need those extra features.
The TSC2008I deviates from the group in that it still shares that same footprint and pinout for the fundamental TSC interface but it has a more advanced register controlled internal state machine based set of TSC functions that shift some load off of the processor and into the chip. So this variant could offer the opportunity to drop it in and use it to off-load some processor overhead. I don’t think this would be supported by the driver library. However, if push comes to shove in the years to come and the older string of parts becomes more scarce or we need to recover some processor bandwidth then this could provide a (software revision only) hardware compatible drop-in alternative.
I am focusing on the Texas Instruments P/N TSC2046EIRGV TSC as the primary redesign target device.
Presently there is deep stock (>25K pcs).
This “E” version part has improved electrostatic hardening.
To extend the SPI interface to it...
I would use one of the orphaned I2C-SCL/ SDA signals to implement the /CS signal.
There is a BUSY signal that does not show up on the TSC2008 and I am not sure if it is needed on the TSC2046. The driver we would plan to use does not reference the BUSY signal.
I probably could support it using the remaining orphaned SCL/SDA signal but I’d rather not have to because, due to circuit density issues, I may need to repurpose that trace path on the board to extend the MISO signal to the TSC chip location.
Here is how I think it would all map into the board based on the pin-out of the VQFN-16 package:
3.3V to +Vcc (5), IOVDD (14)
Touch Screen to (6,7,8,9)
SPI-MISO to D-out (16)
SPI-MOSI to D-in (2)
SPI-CLK to DCLK (4)
I2C SCL or I2C SDA to /CS (3)
I2C-SDA repurposed to carry MISO or to BUSY (1)
TOUCH-IRQ to /PENIRQ (15)
GND to GND (10), Vbat (11), AUX (12)
N.C. to Vref (13)
Do you think this is a valid approach for a long term solution.
