RFC: phy: restore operation at lower (50MHz) sys-clk-freq #33
Conversation
Following commit 15bc2db, LiteSDCard no longer works properly at the low end of system clock speeds (e.g., 50MHz). Reverting the way `self.ce` and `self.clk` are generated to `comb` instead of `sync`, allows LiteSDCard to operate at the full previous range of system clock speeds. Fixes: 15bc2db Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
|
|
||
| # Ensure we don't get short pulses on the SDCard Clk. | ||
| ce_delayed = Signal() | ||
| ce_latched = Signal() | ||
| self.sync += If(clk_d, ce_delayed.eq(self.clk_en)) | ||
| self.comb += If(clk_d, ce_latched.eq(self.clk_en)).Else(ce_latched.eq(ce_delayed)) | ||
| self.sync += self.clk.eq(~clk & ce_latched) | ||
| self.comb += self.clk.eq(~clk & ce_latched) |
There was a problem hiding this comment.
@enjoy-digital : I've understood and agree with all the rest of the changes in commits 1a3d474 and 15bc2db, but not sure why self.ce and self.clk were switched to sync from the original comb. It is the reason sdcard breaks at lower (50MHz) system clock frequencies.
To be honest, though, I also don't get why this change breaks sdcard at low speeds :)
|
@enjoy-digital : ping? |
|
Hi @gsomlo, sorry for the delay and thanks for investigating. The reason for the change was to improve the timings (enjoy-digital/litex#1814). I'll have a closer look at this. |
The weird thing is that if one switches |
Following commit 15bc2db, LiteSDCard no longer works properly at the low end of system clock speeds (e.g., 50MHz). Reverting the way
self.ceandself.clkare generated tocombinstead ofsync, allows LiteSDCard to operate at the full previous range of system clock speeds.Fixes: 15bc2db
tested on nexys_video with vexriscv at 50 and 100 MHz, and with rocket at 50MHz on the nexys_video and ecpix5.