2A-34.Address Decoding

Theory of Operation (2635A) 2A

Detailed Circuit Description

The four chip-select outputs on the Microprocessor are individual software programmed elements that allow the Microprocessor to select the base address, the size, and the number of wait states for the memory accessed by each output.

The FLASH* signal (A1U1-128) enables accesses to 128 kilobytes of Flash Memory (A1U14 and A1U16). The FLASH* signal goes through jumper W3, which must always be installed during normal instrument operation. W3 is removed only during the initial programming of the Flash Memory during production at the factory. The SRAM* signal enables the Nonvolatile Static RAM (A1U20 and A1U24), and the MCARD* signal goes to the Memory Card Interface PCA (A6). The I/O* signal goes to the I/O Decoder (A1U11), which decodes small areas of address space for I/O devices like the FPGA, the Real-Time Clock, and the Option Interface. There are no wait states for accesses to FLASH* and SRAM*, but two wait states are used for any access to I/O*. Each wait state adds approximately 83 nanoseconds to the length of a memory read or write cycle. The Memory Card Interface handles wait state timing for any accesses to MCARD*.

When the Microprocessor is starting up (also referred to as "booting"), the address decoding maps the address space as shown in Table 2A-2.

Table 2A-2. Booting Microprocessor Memory Map (2635A)

Hexadecimal Address

000000 - 03FFFF

100000 - 13FFFF

300000 - 30007F

300080 - 3000FF

300100 - 30017F

310000 - 311FFF

400000 - 401000

Device Selected

Flash (A1U14 and A1U16)

NVRAM (A1U20 and A1U24)

FPGA Configuration (A1U25)

Real-Time Clock (A1U12)

Option Interface (A1J1)

Memory Card Interface (A1P4)

Microprocessor Internal

Just before beginning execution of the instrument code, the address decoding is changed to map the address space as shown in Table 2A-3. This change switches the positions of Flash Memory and Nonvolatile Static RAM within the address space of the Microprocessor.

Table 2A-3. Instrument Microprocessor Memory Map (2635A)

Hexadecimal Address

000000 - 03FFFF

100000 - 13FFFF

300000 - 300007

300008 - 30000F

300010 - 300017

300018 - 30001F (Read Only)

300020 - 300027 (Read Only)

300080 - 3000FF

300100 - 30017F

310000 - 311FFF

400000 - 401000

Device Selected

NVRAM (A1U20 and A1U24)

Flash (A1U14 and A1U16)

FPGA Control / Status (A1U25)

Alarm Outputs (A1U25)

Digital Outputs (A1U25)

Digital Inputs (A1U25)

Keyboard Input (A1U25)

Real-Time Clock (A1U12)

Option Interface (A1J1)

Memory Card Interface (A1P4)

Microprocessor Internal

2A-13

2635A, 2625A specifications

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Fluke 2625A, 2635A service manual 2A-34.Address Decoding

Models: 2635A 2625A