Communication between SoC and FPGA

This document describes how FPGA and SoC communicate with each other in TwPM project for NEORV32. State presented here is valid for release v0.2.0 of top module.

Memory map

Address	Size	Name	Access
0xF0000000	4B	STATUS	RO
0xF0000004	4B	OP_TYPE	RO
0xF0000008	4B	LOCALITY	RO
0xF000000C	4B	BUF_SIZE	RO
0xF0000040	4B	COMPLETE	WO
0xF0000800	2KiB	FPGA_RAM	RW

Reads from invalid addresses in 0xF0000000-0xF0000FFF range return 0xBADFABAC (BAD FABric ACcess), writes are dropped. Most values are valid only if exec bit in STATUS register is set.

Registers

Reserved bits are read as 0. They may change in the future.

STATUS

• exec indicates that MCU is expected to act upon request sent from FPGA, specified in OP_TYPE register. Interrupt is generated when this signal gets set by FPGA.
• abort is set when PC aborts command currently being executed by TPM.
• complete is a signal sent towards FPGA by writing to COMPLETE register. The signal is active for a few cycles after write to that register, after that is should be automatically set back to 0.

OP_TYPE

• op_type is a type of operation expected from MCU. 0 is used as a default value to which this register returns after complete signal is acknowledged by FPGA, code should treat this value as error because no valid path produces it when requesting interaction from MCU. 0xC is reserved as it may be part of 0xBADFABAC magic value. This register is only valid if exec is set.

op_type	Operation
0x0	No operation
0x1	Execute TPM2 command located in FPGA_RAM
0xC	Illegal operation type
other	Reserved for future use

LOCALITY

• locality specifies the locality at which the current operation was initiated. Value 0xF is used when no locality was active at that point. This field is only valid if exec is set.

BUF_SIZE

• buf_size specifies size (in bytes) of data available in FPGA RAM, which is usually a TPM2 command. This field is only valid if exec is set. TPM stack is not expected to write size of response to this field, it is deduced from response's payload.

COMPLETE

Any write to this register triggers complete signal to be sent to FPGA. Reads from this register aren't implemented, meaning that 0xBADFABAC is returned.

FPGA RAM

This region behaves like RAM if exec is set. Its content depends on op_type, which in most cases will be TPM2 command. Response is written by MCU to this buffer, starting from offset 0, overwriting the command.

When exec is not set, data read from this region is invalid and writes are dropped.

Interrupts

TwPM needs to communicate from FPGA towards SoC when there is an operation to be performed by TPM stack. This is done through mext_irq_i, which is responsible for signaling Machine external interrupt (MEI). This signal is level-triggered and high-active. MEI has a dedicated value in mcause, shared between all external interrupts. The exact reason has to be obtained in a platform-specific way; in case of TwPM it can be read from STATUS register, however, as of now only exec generates an interrupt so this check is redundant.

The interrupt signal remains active until SoC signals completion by performing a write to COMPLETE register. For this reason, the IRQ must be masked when TPM command is being executed outside of interrupt service routine (ISR).