hwconfig: Fix pad config for SD SPI, move clockrate to hwconfig

The code snippet in hwconfig to adjust the drive strength was incorrect:
It was adjusting the wrong pins. This was probably not updated during
some pin shuffling in the breadboard phase.

Fix this by adjusting the correct pins. Experimentation shows that both
setting the slew rate to fast or setting the drive strength to 8 mA
(default: slow and 4 mA) is sufficient to make SD SPI work at 25 MHz on
the Rev1 PCB. For now, the combination 8 mA and slow is chosen (slow
slew rate should result in less high frequency EMI).

Also make the SD clock rate adjustable in hwconfig, and set it to 25 MHz
for Rev1 and 15 MHz for the breadboard setup.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

software/src/hwconfig_Rev1.py

@@ -9,6 +9,7 @@ SD_DI = Pin.board.GP3
 SD_DO = Pin.board.GP4
 SD_SCK = Pin.board.GP2
 SD_CS = Pin.board.GP5
+SD_CLOCKRATE = 25000000
 
 # MAX98357
 I2S_LRCLK = Pin.board.GP6
@@ -46,10 +47,11 @@ def board_init():
     POWER_EN.init(mode=Pin.OUT)
     POWER_EN.value(1)
 
-    # Set 8 mA drive strength and fast slew rate for SD SPI
-    machine.mem32[0x4001c004 + 6*4] = 0x67
-    machine.mem32[0x4001c004 + 7*4] = 0x67
-    machine.mem32[0x4001c004 + 8*4] = 0x67
+    # SD_DO / MISO input doesn't need any special configuration
+    # Set 8 mA drive strength for SCK and MOSI
+    machine.mem32[0x4001c004 + 2*4] = 0x60  # SCK
+    machine.mem32[0x4001c004 + 3*4] = 0x60  # MOSI
+    # SD_CS doesn't need any special configuration
 
     # Permanently enable amplifier
     # TODO: Implement amplifier power management

software/src/hwconfig_breadboard.py

@@ -1,7 +1,6 @@
 # SPDX-License-Identifier: MIT
 # Copyright (c) 2025 Matthias Blankertz <matthias@blankertz.org>
 
-import machine
 from machine import Pin
 
 # SD Card SPI
@@ -9,6 +8,7 @@ SD_DI = Pin.board.GP3
 SD_DO = Pin.board.GP4
 SD_SCK = Pin.board.GP2
 SD_CS = Pin.board.GP5
+SD_CLOCKRATE = 15000000
 
 # MAX98357
 I2S_LRCLK = Pin.board.GP7
@@ -41,10 +41,7 @@ VBAT_ADC = Pin.board.GP26
 
 
 def board_init():
-    # Set 8 mA drive strength and fast slew rate for SD SPI
-    machine.mem32[0x4001c004 + 6*4] = 0x67
-    machine.mem32[0x4001c004 + 7*4] = 0x67
-    machine.mem32[0x4001c004 + 8*4] = 0x67
+    pass
 
 
 def get_battery_voltage():

software/src/main.py

@@ -62,7 +62,7 @@ def run():
 
     # Setup MP3 player
     with SDContext(mosi=hwconfig.SD_DI, miso=hwconfig.SD_DO, sck=hwconfig.SD_SCK, ss=hwconfig.SD_CS,
-                   baudrate=15000000), \
+                   baudrate=hwconfig.SD_CLOCKRATE), \
          AudioContext(hwconfig.I2S_DIN, hwconfig.I2S_DCLK, hwconfig.I2S_LRCLK) as audioctx:
 
         # Setup NFC