Using OTP Memories To Keep SoC Power Down

Virtually all SoCs require one-time programmable (OTP) memory. Each SoC is different, of course, but two main uses are large memories for holding boot and programming code and small memories for holding encryption keys and trimming parameters, such as radio tuning information and so on.

There are alternatives to putting an OTP on-chip. The data can be held off-chip in some sort of programmable memory (or, perhaps, ROM). But this obviously has the disadvantage of requiring the cost of an extra chip. In smartphones it is not just the cost of another chip that is a problem, but the additional volume taken up by two chips. There is just not a lot of room inside a smartphone to fit everything.

Another alternative to OTP memory is flash memory. This has a big advantage, which is that a flash memory can be reprogrammed many times. However, this comes with a big disadvantage in terms of added process complexity and, thus, the cost of the silicon. Even when off-chip flash memory already exists, security reasons may make using it for holding critical data impractical and running code out of flash memory may, in fact, require data from the flash to be copied to SRAM on the chip, which is both an added cost and yet another increase of unwanted power.

OTP memory has the advantage that code can be executed in-place and does not need to be copied from external memory into on-chip SRAM. It is fast enough and with low enough power as to make copying data out to SRAM something that is unnecessary.

The Sidense one-transistor OTP (1T-OTP) architecture is especially area efficient since it uses a single transistor per bit cell. Furthermore, it does not depend on charge storage and so once programmed, it cannot be un-programmed by environmental or electrical upsets. The patented Sidense 1T-Fuse™ antifuse technology works by permanently rupturing the gate-oxide under the bit-cell’s storage transistor in a controlled fashion, obviously something irreversible.

Another big advantage of the Sidense antifuse approach is that it uses an unmodified digital process. No additional masks or process steps are required, so nothing is added to the wafer manufacturing cost. The per-chip cost rises due to the area occupied by the OTP, but since the 1T-OTP macros are very area-efficient this increase is usually very small. Additionally if the 1T-OTP is programmed at the tester, the increase in test time will also result in some extra cost.

The Sidense 1T-OTP memory uses a low read voltage, which further keeps the power of the memory down. The Sidense memory does require some non-standard voltages internally, especially during programming, but these are created using embedded charge pumps and are hidden from the user. The OTP memory can simply be hooked up to the chip’s power supply network just like any other memory block.

Another option to the Sidense solution to lower the power even more is to use differential bit storage. This technique requires each bit of information to be represented using two transistors: one 0 and one 1. This makes sensing the state simpler and as a result the voltage required for the memory can be lower still, along with the associated power. Obviously this comes at the cost of an increase in area since the number of transistors required to represent a given amount of data is doubled within the memory macro.

Read the white paper Using Sidense 1T-OTP in Power-sensitive Applications here.