constraints comments about DAC output strength for earphones

doc/constraints/ulx3s_v17patch.lpf

@@ -138,8 +138,9 @@ IOBUF  PORT "adc_sclk" PULLMODE=UP IO_TYPE=LVCMOS33 DRIVE=4;
 # 4-bit mode can drive down to 75 ohm load impedance.
 # Lower impedance leads to IO overload, 
 # FPGA will stop working and need reboot.
-# For standard 17 ohm earphones:
+# For standard 16 ohm earphones on PCB v1.7:
 # use bits 2,3 as input (High-Z) and drive only bits 0,1.
+# PCB v2.1.2 and higher can use full 4 bits and 16 mA drive for 16 ohm earphones.
 LOCATE COMP "audio_l[3]" SITE "B3"; # JACK TIP (left audio)
 LOCATE COMP "audio_l[2]" SITE "C3";
 LOCATE COMP "audio_l[1]" SITE "D3";

doc/constraints/ulx3s_v20.lpf

@@ -138,12 +138,8 @@ IOBUF  PORT "adc_miso" PULLMODE=UP IO_TYPE=LVCMOS33 DRIVE=4;
 IOBUF  PORT "adc_sclk" PULLMODE=UP IO_TYPE=LVCMOS33 DRIVE=4;
 
 ## Audio 4-bit DAC "analog", "gpio" sheet
-# 4-bit mode can drive down to 75 ohm load impedance.
+# Output impedance 75 ohm.
-# Lower impedance leads to IO overload, 
+# Strong enough to drive 16 ohm earphones.
-# FPGA will stop working and need reboot.
-# For standard 17 ohm earphones on PCB v1.7:
-# use bits 2,3 as input (High-Z) and drive only bits 0,1.
-# PCB v2.1.2 can use full 4 bits and 16mA drive for 17 ohm earphones.
 LOCATE COMP "audio_l[3]" SITE "B3"; # JACK TIP (left audio)
 LOCATE COMP "audio_l[2]" SITE "C3";
 LOCATE COMP "audio_l[1]" SITE "D3";