STM32F4 8-bit support for 1.8v operation

Per ST doc DM00035129.pdf "DocID022063 Rev 5", the STM32F415xx data sheet,
table 40 on page 110 of the PDF, 32-bit program operation is only possible
above 2.7 Volts.

In order to support programming on devices running at lower voltages, this
commit adds an 8-bit programming mode and the necessary tests to enable it
when the reported voltage is below 2.7 Volts.

flashloaders/stm32f4lv.s

@@ -0,0 +1,33 @@
+.global start
+.syntax unified
+
+@ r0 = source
+@ r1 = target
+@ r2 = wordcount
+@ r3 = flash_base
+@ r4 = temp
+
+start:
+    lsls    r2, r2, #2
+    ldr     r3, flash_base
+next:
+    cbz     r2, done
+    ldrb    r4, [r0]
+    strb    r4, [r1]
+
+wait:
+    ldrh    r4, [r3, #0x0e]
+    tst.w   r4, #1
+    bne     wait
+
+    add     r0, #1
+    add     r1, #1
+    sub     r2, #1
+    b       next
+done:
+    bkpt
+
+.align 2
+
+flash_base:
+	.word 0x40023c00

src/stlink-common.c

@@ -65,6 +65,7 @@
 
 //STM32L0x flash register base and offsets
 //same as 32L1 above
+// RM0090 - DM00031020.pdf
 #define STM32L0_FLASH_REGS_ADDR ((uint32_t)0x40022000)
 #define FLASH_ACR_OFF     ((uint32_t) 0x00)
 #define FLASH_PECR_OFF    ((uint32_t) 0x04)
@@ -1374,6 +1375,29 @@ int write_loader_to_sram(stlink_t *sl, stm32_addr_t* addr, size_t* size) {
         0x00, 0x3c, 0x02, 0x40,
     };
 
+    static const uint8_t loader_code_stm32f4_lv[] = {
+        // flashloaders/stm32f4lv.s
+        0x92, 0x00,
+
+        0x08, 0x4b,
+        0x62, 0xb1,
+        0x04, 0x78,
+        0x0c, 0x70,
+
+        0xdc, 0x89,
+        0x14, 0xf0, 0x01, 0x0f,
+        0xfb, 0xd1,
+        0x00, 0xf1, 0x01, 0x00,
+        0x01, 0xf1, 0x01, 0x01,
+        0xa2, 0xf1, 0x01, 0x02,
+        0xf1, 0xe7,
+
+        0x00, 0xbe,
+        0x00, 0xbf,
+
+        0x00, 0x3c, 0x02, 0x40,
+    };
+
     const uint8_t* loader_code;
     size_t loader_size;
 
@@ -1391,15 +1415,21 @@ int write_loader_to_sram(stlink_t *sl, stm32_addr_t* addr, size_t* size) {
         loader_size = sizeof(loader_code_stm32vl);
     } else if (sl->chip_id == STM32_CHIPID_F2 || sl->chip_id == STM32_CHIPID_F4 || (sl->chip_id == STM32_CHIPID_F4_DE) ||
             sl->chip_id == STM32_CHIPID_F4_LP || sl->chip_id == STM32_CHIPID_F4_HD || (sl->chip_id == STM32_CHIPID_F411RE)){
-        loader_code = loader_code_stm32f4;
-        loader_size = sizeof(loader_code_stm32f4);
+        int voltage = stlink_target_voltage(sl);
+        if (voltage > 2700) {
+            loader_code = loader_code_stm32f4;
+            loader_size = sizeof(loader_code_stm32f4);
+        } else {
+            loader_code = loader_code_stm32f4_lv;
+            loader_size = sizeof(loader_code_stm32f4_lv);
+        }
     } else if (sl->chip_id == STM32_CHIPID_F0 || sl->chip_id == STM32_CHIPID_F04 || sl->chip_id == STM32_CHIPID_F0_CAN || sl->chip_id == STM32_CHIPID_F0_SMALL || sl->chip_id == STM32_CHIPID_F09X) {
         loader_code = loader_code_stm32f0;
         loader_size = sizeof(loader_code_stm32f0);
     } else if (sl->chip_id == STM32_CHIPID_L0) {
-		loader_code = loader_code_stm32l0;
-		loader_size = sizeof(loader_code_stm32l0);
-	} else {
+        loader_code = loader_code_stm32l0;
+        loader_size = sizeof(loader_code_stm32l0);
+    } else {
         ELOG("unknown coreid, not sure what flash loader to use, aborting!: %x\n", sl->core_id);
         return -1;
     }
@@ -1588,7 +1618,14 @@ int stlink_write_flash(stlink_t *sl, stm32_addr_t addr, uint8_t* base, uint32_t
 
         /* TODO: Check that Voltage range is 2.7 - 3.6 V */
         /* set parallelisim to 32 bit*/
-        write_flash_cr_psiz(sl, 2);
+        int voltage = stlink_target_voltage(sl);
+        if (voltage > 2700) {
+            printf("enabling 32-bit flash writes\n");
+            write_flash_cr_psiz(sl, 2);
+        } else {
+            printf("Target voltage (%d mV) too low for 32-bit flash, using 8-bit flash writes\n", voltage);
+            write_flash_cr_psiz(sl, 0);
+        }
 
         /* set programming mode */
         set_flash_cr_pg(sl);