HTC's onslaught of 64-bit smartphones for this fall will continue with the Desire 820; however, new rumors point to the device not using an octa-core 64-bit Snapdragon anymore. That octa-core chip could not be any other than the ARM Cortex A53-based Snapdragon 615, Qualcomm's only 64-bit/ARMv8 chip that has eight cores. Right now, though, it seems HTC may be switching to the quad-core Snapdragon 410, the same chip it will have in the recently announced Desire 510.
Since the 615 isn't even a typical big.Little chip that maximizes energy efficiency at the low-end and maximizes performance at the high-end, it looks like the the octa-core Snapdragon 615 exists mainly for marketing purposes and not much else. It uses twice as many cores as Snapdragon 410, which increases the price significantly. HTC may have thought it's not worth the extra cost. The company could also get a bigger volume discount by buying more Snapdragon 410 chips.
The downside of such a move is that HTC is not only cutting the number of CPU cores going into the Desire 820 but also the clock speed, going from 1.7 GHz for the Snapdragon 615 to 1.2 GHz for the Snapdragon 410. The GPU is also downgraded from the more powerful Adreno 405 that supports OpenGL ES 3.1 to Adreno 306 that only supports OpenGL ES 3.0.
The Desire 820 is rumored to have a 5.5" display, 720p resolution (which tells us it won't cost too much), 13MP rear camera, 8MP front camera (which would probably make it one of the best "selfie" cameras on the market), 1.5 GB of RAM, 8 GB of storage and a 2,600 mAh battery.
Keep in mind this is a rumor, and HTC may still have two versions of the Desire 820: one international and one for the Chinese market. Since many-core chips seem to be more preferred in China, the device could still get the eight-core Snapdragon 615. However, if that version still exists, it may arrive a little later. The Snapdragon 615 is a 20 nm chip, while Snapdragon 410 is 28 nm, so it will only be released once Qualcomm can begin using the 20 nm process on whichever foundry is making its chips.
HTC is making the announcement next Thursday, so we'll then know for sure which chip the Desire 820 will be using.