AN1176 STMicroelectronics, AN1176 Datasheet - Page 3

• Manufacturer Part Number: AN1176
• Description: 68HC11/PSD813F1 DESIGN GUIDE
• Manufacturer: STMicroelectronics

1 Introduction
FLASH PSD813F devices are members of a family of flash-based peripherals for use with
embedded microcontrollers (MCUs). These programmable system devices (PSDs) consist of
memory, logic, and I/O. When coupled with a low-cost, ROM-less 68HC11 MCU, the PSD
forms a complete embedded flash system that is 100% In-System-Programmable (ISP). There
are many features in the PSD silicon and in the PSDsoft development software that makes ISP
easy for you, regardless of how much experience you have in embedded flash design.
This document will offer two flash 68HC11 designs using a PSD813F device. The first is a
simple system to get up and running quickly for basic applications, or to check out your
prototype 68HC11 hardware. The second design illustrates the use of enhanced features of PSD
In-System-Programming as applied to the 68HC11. You can start with the first design, and
migrate to the second as your functional requirements grow.
The Generic Problem
Typically, a host computer downloads firmware into an embedded flash system through a
communication channel that is controlled by the MCU. This channel is usually a UART, but any
communication channel that the 68HC11 supports will do (network, SPI, CAN, J1850, etc.). The
68HC11 must execute the code that controls the ISP process from an independent memory array
that is not being erased or programmed. Otherwise, boot code and flash programming algorithms
(ISP loader code) will be unavailable to the 68HC11. It is absolutely necessary to use an
alternate memory array (an independent memory that is not being programmed) to store the ISP
loader code.
A system designer must choose the type of alternate memory to store ISP loader code (ROM,
SRAM, FLASH, or EEPROM); each type has advantages and disadvantages. This alternate
memory may reside external to the MCU, or reside on-board the MCU. A top-level view of an
embedded ISP flash system with external memory is shown in Figure 1.
Page
C om puter
H ost
1
Com m unication
Figure 1 – Embedded flash system capable of ISP (5 devices)
Channel
Em bedded System
68HC 11
C P LD
for IS P Lo a d er C o d e
M a in F la sh M e m ory
A ltern a te M e m ory
S ystem S R A M
1 2 8K B ytes
8 K B yte s
S ystem
I/O