EZ-USB Datasheet, PDF (76/334 Pages) Cypress Semiconductor – The EZ-USB USB Integrated Circuit

5 EZ-USB Enumeration and ReNumeration™
5.1 Introduction
The EZ-USB chip is soft. 8051 code and data is stored in internal RAM, which is loaded
from the host using the USB interface. Peripheral devices that use the EZ-USB chip can
operate without ROM, EPROM, or FLASH memory, shortening production lead times
and making firmware updates a breeze.
To support the soft feature, the EZ-USB chip automatically enumerates as a USB device
without firmware, so the USB interface itself may be used to download 8051 code and
descriptor tables. The EZ-USB core performs this initial (power-on) enumeration and
code download while the 8051 is held in reset. This initial USB device, which supports
code download, is called the “Default USB Device.”
After the code descriptor tables have been downloaded from the host to EZ-USB RAM,
the 8051 is brought out of reset and begins executing the device code. The EZ-USB
device enumerates again, this time as the loaded device. This second enumeration is
called “ReNumeration™,” which the EZ-USB chip accomplishes by electrically simulat-
ing a physical disconnection and re-connection to the USB.
An EZ-USB control bit called “ReNum” (ReNumerated) determines which entity, the core
or the 8051, handles device requests over endpoint zero. At power-on, the RENUM bit
(USBCS.1) is zero, indicating that the EZ-USB core automatically handles device
requests. Once the 8051 is running, it can set ReNum=1 to indicate that user 8051 code
handles subsequent device requests using its downloaded firmware. Chapter 7, "EZ-USB
Endpoint Zero" describes how the 8051 handles device requests while ReNum=1.
It is also possible for the 8051 to run with ReNum=0 and have the EZ-USB core handle
certain endpoint zero requests (see the text box, “Another Use for the Default USB
Device” on page 5-2).
This chapter deals with the various EZ-USB startup modes, and describes the default USB
device that is created at initial enumeration.
EZ-USB TRM v1.9
Chapter 5. EZ-USB CPU
Page 5-1