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Typo in readme

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Luke Wren 2024-08-07 22:22:05 -07:00
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Readme.md
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@ -302,14 +302,14 @@ info reg a0
There is a tiny [example SoC](example_soc/soc/example_soc.v) which builds on both iCEBreaker and ULX3S. The SoC contains:
- A Hazard3 processor, in a single-ported RV32IM configuration, with debug support
- A Hazard3 processor, in a single-ported RV32IMA configuration, with debug support
- A Debug Transport Module and Debug Module to access Hazard3's debug interface
- 128 kB of RAM (fits in UP5k SPRAMs)
- A UART
On iCEBreaker (a iCE40 UP5k development board), the processor can be debugged using the onboard FT2232H bridge, through a standard RISCV-V JTAG-DTM exposed on four IO pins. Connecting JTAG requires two solder jumpers to be bridged on the back to connect the JTAG -- see the comments in the [pin constraints file](example_soc/synth/fpga_icebreaker.pcf). FT2232H is a dual-channel FTDI device, so the UART and JTAG can be accessed simultaneously for a very civilised debug experience, with JTAG running at the full 30 MHz supported by the FTDI.
ULX3S is based on a much larger ECP5 FPGA. Thanks to [this ECP5 JTAG adapter](hdl/debug/dtm/hazard3_ecp5_jtag_dtm.v), it is possible to attach the guts of a RISC-V JTAG-DTM to the custom DR hooks in ECP5's chip TAP. With the right config file you can then convince OpenOCD that the FPGA's own TAP *is* a JTAG-DTM. You can debug Hazard3 on ULX3S using the same micro USB cable you use to load the bitstream, no soldering required. The downside is that the FT231X device on the ULX3S is actually a UART bridge which supports JTAG by bitbanging the auxiliary UART signals, which is incredibly slow. The UART can not be used simultaneously with JTAG access.
ULX3S is based on a much larger ECP5 FPGA. Thanks to [this ECP5 JTAG adapter](hdl/debug/dtm/hazard3_ecp5_jtag_dtm.v), it is possible to attach the guts of a RISC-V JTAG-DTM to the custom DR hooks in ECP5's chip TAP. With the right config file you can then convince OpenOCD that the FPGA's own TAP *is* a JTAG-DTM. You can debug Hazard3 on ULX3S using the same micro USB cable you use to load the bitstream, no soldering required. The downside is that the FT231X device on the ULX3S is actually a UART bridge which supports JTAG by bitbanging the auxiliary UART signals, which is incredibly slow. The UART cannot be used simultaneously with JTAG access.
For these reasons -- much faster JTAG, and simultaneous UART access -- iCEBreaker is currently a more pleasant platform to debug if you don't have any external JTAG probe.