diff --git a/docs/stm32.md b/docs/stm32.md
index 3900946..fcdbd5b 100644
--- a/docs/stm32.md
+++ b/docs/stm32.md
@@ -1,14 +1,14 @@
 ### stm32
 
 menu items:
-- MENU_XSERIAL: ```generic``` (```none```, ```disabled```)
-- MENU_USB: ```CDCgen``` (```none```, ```CDC```, ```HID```)
-- MENU_XUSB: ```FS``` (```HS```, ```HSFS```)
-- MENU_OPT: ```osstd``` (```oslto```, ```o1std```, ```o1lto```, ```o2std```, ```o2lto```, ```o3std```, ```o3lto```, ```ogstd```, ```o0std```)
-- MENU_DBG: ```none``` (```enable_sym```, ```enable_log```, ```enable_all```)
-- MENU_RTLIB: ```nano``` (```nanofp```, ```nanofs```, ```nanofps```, ```full```)
-- MENU_UPLOAD_METHOD: ```dfu2Method``` (```swdMethod```, ```dfuMethod```, ```bmpMethod```, ```hidMethod```, ```serialMethod```, ```dfuoMethod```)
-- MENU_SERIAL_PORT: $(SERIAL_PORT) or ```/dev/ttyUSB0```
+- PNUM: _mandatory_
+- XSERIAL: ```generic``` (```none```, ```disabled```)
+- USB: ```CDCgen``` (```none```, ```CDC```, ```HID```)
+- XUSB: ```FS``` (```HS```, ```HSFS```)
+- OPT: ```osstd``` (```oslto```, ```o1std```, ```o1lto```, ```o2std```, ```o2lto```, ```o3std```, ```o3lto```, ```ogstd```, ```o0std```)
+- DBG: ```none``` (```enable_sym```, ```enable_log```, ```enable_all```)
+- RTLIB: ```nano``` (```nanofp```, ```nanofs```, ```nanofps```, ```full```)
+- UPLOAD_METHOD: ```dfu2Method``` (```swdMethod```, ```dfuMethod```, ```bmpMethod```, ```hidMethod```, ```serialMethod```, ```dfuoMethod```)
 
 other variables:
 - SERIAL_PORT: ```/dev/ttyACM0```
@@ -18,7 +18,8 @@ examples:
 
 [HY-TINY](https://stm32duinoforum.com/forum/wiki_subdomain/index_title_HY-TinySTM103T.html):
 ```
-BOARD := GenF1.menu.pnum.HY_TINYSTM103TB
+BOARD := GenF1
+PNUM := HY_TINYSTM103TB
 include stm32.mk
 
 ```
diff --git a/stm32.mk b/stm32.mk
index 66a104e..fbb9ebc 100644
--- a/stm32.mk
+++ b/stm32.mk
@@ -1,12 +1,11 @@
-VENDOR := STMicroelectronics
-PROCESSOR_FAMILY := stm32
+ifndef PNUM
+$(error PNUM required)
+endif
 
 SERIAL_PORT ?= /dev/ttyACM0
 TERMINAL_SPEED ?= 115200
 STM_TOOLS ?= /usr/local
 
-export PATH := $(PATH):$(STM_TOOLS)/STMicroelectronics/STM32Cube/STM32CubeProgrammer/bin
-
 # menus
 XSERIAL ?= generic
 USB ?= CDCgen
@@ -16,11 +15,15 @@ DBG ?= none
 RTLIB ?= nano
 UPLOAD_METHOD ?= dfu2Method
 
+VENDOR := STMicroelectronics
+PROCESSOR_FAMILY := stm32
+
+export PATH := $(PATH):$(STM_TOOLS)/STMicroelectronics/STM32Cube/STM32CubeProgrammer/bin
+
 -include hardware.mk
 
-FAMILY := $(firstword $(subst ., ,$(BOARD)))
-build.bootloader_flags := $($(FAMILY).menu.upload_method.$(UPLOAD_METHOD).build.bootloader_flags)
-build.st_extra_flags := $($(FAMILY).build.st_extra_flags)
+build.bootloader_flags := $($(BOARD).menu.upload_method.$(UPLOAD_METHOD).build.bootloader_flags)
+build.st_extra_flags := $($(BOARD).build.st_extra_flags)
 
 LIBRARIES += SrcWrapper