STM32L USB Driver for Keil

The USB device engine on STM32L is almost same as STM32F103.
The only difference is the D+ pull-up resistor: STM32L has on-chip pull-up.
But this pull-up isn't compliant to the USB spec, as this errata tells, on all revision.

STM32L15xx errata sheet
www.st.com/.../CD00278726.pdf
2.5.1 Pull-up resistor has a value lower than 1.5kOhm
Description: The pull-up resistor on USB line is requested to have a nominal value of 1.5kOhm, whereas the characterization of the internal pull-up shows a value between 0.81 and 0.95 kOhm o the full temperature range.
Workaround: An external pull-up resistor controlled by a GPIO can be used.

After all, STM32L requires an external pull-up resistor. In this configuration, there is no difference from STM32F103.

Therefore, you may start with usbd_STM32F103.c for STM32F103

Tsuneo

I am not satisfied with the support that Keil gives in this particular area.
Long time ago (16 Nov 2010) they claimed to support this ST-line
(see http://www.keil.com/pr/article/1225.htm)
but as of today, there is still no more than just a blinky sample and no USB at all.

best regards
Uli

homepage2.nifty.com/.../STM32L_DISC_KEIL_RL_HID.zip

A couple of notes of porting,

A) Hardware - USB connector
STM32L-DISCOVERY doesn't have USB receptacle. A USB B type receptacle is connected as follows. Also, an external 1k5 D+ pull-up resistor is placed across PC12 and D+. As mentioned on above post, the on-chip pull-up is not recommended.

 STM32L               USB
DISCOVERY          receptacle

 PA11 --------------- D-

 PA12 -----------+--- D+
                 |
 PC12 --- 1k5 ---+

 GND  --------------- GND

B) Hardware - Crystal
STM32L-DISCOVERY board has pads for a crystal, but not populated.
These components are added.
- X3: 8 MHz crystal
- C21, C22: 22pF
- R30: 220 Ohm

The external crystal is required, because the tolerance of HSI OSC (+/-1%) is not enough for USB operation.

C) Firmware - porting steps
1) Start on MCBSTM32 RL-USB HID example C:\Keil\ARM\Boards\Keil\MCBSTM32\RL\USB\Device\HID

2) Replace "startup" and "system" file with those of STM32L, on Blinky example.
(C:\Keil\ARM\Boards\ST\STM32L-Discovery\Blinky)
- startup_stm32f10x_md.s --> startup_stm32l1xx_md.s
- system_stm32f10x.c --> system_stm32l1xx.c

Fortunately, Blinky "system" file has been already tuned for USB operation (8MHz crystal - HSE OSC and 96MHz PLL), without any change.

3) Modify usbd_STM32F103.c for STM32L differences.
USB Interrupt
- USBD_IntrEna() Replace USB_LP_CAN1_RX0_IRQn with USB_LP_IRQn
- USB_LP_CAN1_RX0_IRQHandler() Rename this function name with USB_LP_IRQHandler()

USB Connection port (PC12)
- USBD_Init() Enable GPIO C
- USBD_Connect() Switch PC12 between floating input and push-pull output, to emulate high-side open-drain switch.

4) Change port assignment of LEDs and a switch on usbd_user_hid.c
- usbd_hid_init()
- usbd_hid_get_report()
- usbd_hid_set_report()

5) Replace #include of stm32f10x.h with stm32l1xx.h on USBD_Demo.c, usbd_user_hid.c and usbd_STM32F103.c

6) Delete LCD support
- LCD_4bit.c
- USBD_Demo.c, main()

7) On uVision, Project menu > Options dialog
- Device tab: select STM32L152RB
- C/C++ tab: enter STM32L1XX_MD on Define box
- Debug tab: Use "ST-Link debugger", Setting > Port: JTAG -> SW
- Utilities tab: Use "ST-Link debugger", Setting > Programming algorithm: STM32L15x Low Power Flash

I believe I've listed up all of changes, but to make it sure, compare the source files with a "diff" utility ;-)

Enjoy!

Tsuneo