gps/hardware/ip/max2771_spi/README.md

MAX2771 Three-Wire SPI IP

This directory contains the dedicated MAX2771 serial engine. It is independent of the generic pmod_spi controller and implements the MAX2771 transaction format directly.

Primary references:

• MAX2771 data sheet, Serial Interface and Table 15
• MAX2771 EV kit data sheet and schematic

Protocol

Every command is one 48-SCLK frame while CSN is low:

clocks 0..11   A[11:0], MSB first; A[11:4] are zero
clock 12       R/W: 1 read, 0 write
clocks 13..15  three turnaround clocks
clocks 16..47  D[31:0], MSB first

SCLK idles low. Both sides sample on rising edges and change data on falling edges. SDATA is one bidirectional line, not separate MOSI and MISO signals. For reads, the host releases SDATA on the falling edge after the R/W bit and keeps it as an input through the three turnaround clocks and 32 data clocks.

The divider uses ceiling division so SCLK never exceeds the 4 MHz data-sheet limit:

board       fabric clock  half-period  SCLK       period
Genesys 2   50 MHz        7 cycles     3.571 MHz  280 ns
VCU118      100 MHz       13 cycles    3.846 MHz  260 ns

Both satisfy the 250 ns minimum period. The engine also enforces at least one half-period for CS setup and hold and at least 100 ns between transactions.

MMIO

The TileLink wrapper is at 0x10016000 on both FPGA targets.

Offset	Name	Description
0x00	COMMAND	bits 3:0 address, bit 8 read; write starts a frame
0x04	WRITE_DATA	32-bit register write value
0x08	READ_DATA	last 32-bit register read value
0x0c	STATUS	bit 0 ready, bit 1 busy
0x10	INPUT_CLOCK_HZ	configured board fabric clock
0x14	SCLK_HZ	actual divided serial clock
0x18	HALF_PERIOD	fabric cycles per SCLK half-period
0x1c	IDENT	0x4d415831 (MAX1)

Simulation

The self-checking testbench models the MAX2771 side of the resolved SDATA wire and covers writes, reads, turnaround high impedance, contention, frame order, frame length, CS timing, and both board clock parameters.

make -C hardware/ip/max2771_spi test

This target uses vlog and vsim. Set QUESTA=/path/to/questa when they are not on PATH.