
PSD8XX Family 
PSD835G2
2
1.0
Introduction
(Cont.)
Please refer to the revision block at the end of this 
document for updated information.
The PSD835G2 device offers two methods to program PSD Flash memory while the PSD
is soldered to a circuit board.
J
 In-System Programming (ISP) via JTAG
An IEEE 1149.1 compliant JTAG-ISP interface is included on the PSD enabling the 
entire device (both flash memories, the PLD, and all configuration) to be rapidly 
programmed while soldered to the circuit board. This requires no MCU participation, 
which means the PSD can be programmed anytime, even while completely blank.
The innovative JTAG interface to flash memories is an industry first, solving key 
problems faced by designers and manufacturing houses, such as:
 First time programming – 
How do I get firmware into the flash the very first time
JTAG is the answer, program the PSD while blank with no MCU involvement.
 Inventory build-up of pre-programmed devices – 
How do I maintain an accurate
count of pre-programmed flash memory and PLD devices based on customer 
demand How many and what version JTAG is the answer, build your hardware 
with blank PSDs soldered directly to the board and then custom program just before 
they are shipped to customer. No more labels on chips and no more wasted 
inventory.
 Expensive sockets – 
How do I eliminate the need for expensive and unreliable 
sockets JTAG is the answer. Solder the PSD directly to the circuit board. Program 
first time and subsequent times with JTAG. No need to handle devices and bend the 
fragile leads.
J
 In-Application re-Programming (IAP)
Two independent flash memory arrays are included so the MCU can execute code 
from one memory while erasing and programming the other. Robust product firmware 
updates in the field are possible over any communication channel (CAN, Ethernet, 
UART, J1850, etc) using this unique architecture. Designers are relieved of these 
problems:
 Simultaneous read and write to flash memory – 
How can the MCU program the 
same memory from which it is executing code It cannot. The PSD allows the MCU 
to operate the two flash memories concurrently, reading code from one while erasing 
and programming the other during IAP.
 Complex memory mapping – 
How can I map these two memories efficiently 
A Programmable Decode PLD is embedded in the PSD. The concurrent PSD
memories can be mapped anywhere in MCU address space, segment by segment 
with extremely high address resolution. As an option, the secondary flash memory 
can be swapped out of the system memory map when IAP is complete. A built-in 
page register breaks the MCU address limit.
 Separate program and data space – 
How can I write to flash memory while it 
resides in “program” space during field firmware updates, my 80C51 won’t allow it  
The flash PSD provides means to “reclassify” flash memory as “data” space during 
IAP, then back to “program” space when complete.
PSDsoft – 
ST
’s software development tool – guides you through the design
process step-by-step making it possible to complete an embedded MCU design 
capable of ISP/IAP in just hours. Select your MCU and PSDsoft will take you through 
the remainder of the design with point and click entry, covering...PSD selection, pin 
definitions, programmable logic inputs and outputs, MCU memory map definition, ANSI C
code generation for your MCU, and merging your MCU firmware with the PSD design. 
When complete, two different device programmers are supported directly from PSDsoft –
FlashLINK (JTAG) and PSDpro.
The PSD835G2 is available in an 80-pin TQFP package.