USB Blaster Cheap Clone
=======================
The Altera USB Blaster has a cheap clone, based on :
- a Cypress CY7C68013A-56PVXC as the main chip
- a 74HC244D as the output latch
- a 24 MHz quartz
- a EEPROM 24C64BN
The schematics (cut down to essential) is :
/-----------------+----------------------\
+--------------+ | | |
USB--| CY7C68013A | | +----------+ | |
| | | | 74HC244D | | |
. . | | | | |
. . \--o 1 20 o | 10 pins header |
| 47 o-- TCK --o 2 19 o---/ +-------+ |
| 46 o-- TDO --o 3 18 o-- TCK -----o 1 2 o |
| 45 o-- TMS --o 4 17 o-- TDO -----o 3 4 o |
| 44 o o 5 16 o-- TMS -----o 5 6 o |
| 43 o-- o 6 15 o o 7 8 o |
| 42 o-- o 7 14 o +--o 9 10 o |
| 41 o-- TDI --o 8 13 o-- ? | +-------+ |
. 40 o-- nOE \ o 9 12 o-- TDI --+ |
. . | o 10 11 o |
o 28 29 o | | | |
| | | +----------+ |
+--------------+ \ |
---------------------------------------/
From this one can deduce that :
- the cypress emulates the Altera chip
- as the cypress pins used are 41-47, all output/input are controlled by 8051
PortA.
- as the 8051 is clocked at 24Mhz, and because each USB byte is handled by the
8051, assuming a 40 instruction cycles per USB packet, the maximum throughput
would be around 500 kHz.
Pinout
======
Port A.0: nOE (output enable of 74HC244D)
Port A.1: TDI
Port A.5: TMS
Port A.6: TDO
Port A.7: TCK
Throughput considerations
=========================
Measurements on a scope reveal that :
- for bitbang mode, the throughput is 56.5 kbits/s
(as each clock transition is measured at 17.7us)
- for byteshift mode, the throughput is 107.7 kbits/s
(as 63 bits TDI transmission is measured in 585 us)
Let's suppose that to upload a 32 bits value, it is necessary to :
- move from IDLE to DR-SHIFT : 3 bitbang (3 TMS transitions)
- input the 32 bits of data : 1 byteshift (24 bits) + 8 bitbang (8 bits)
- move from DR-SHIFT to IDLE : 5 bitbang (5 TMS transitions)
So for this 32 bits of data, the time would be :
3 * 17.7us + 1 * 585us/63*24 + 5 * 17.7us
= 53.1us + 222us + 88.5us
= 363us
Throughput in bit/s: 32 * (1 / 363E-6) = 88000 bits/s
Throughput in bytes/s: 11kBytes/s
Conclusion
==========
Contrary to the original USB Blaster, the cheap clone will never reach high
transfer speeds over JTAG.
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