Device Architecture
2- 36
Pr el iminar y v1 .7
an external crystal frequency of 32.768 kHz, the prescaler output clock will toggle at a rate of
32.768 kHz / 128 = 256 Hz.
The RTC is built from and controlled by a set of registers, denoted "Main Registers" in
Figure 2-27The FPGA fabric portion of the Fusion device must be powered up and active at least once to write
to the various registers within the RTC to initialize them for the user’s application. Users set up the
RTC by configuring it from the Actel SmartGen tool, implementing custom logic or programming a
soft microcontroller.
The 40-bit counter and match registers are each divided into five bytes. Each byte is directly
addressable by the ACM. The address map of registers accessed through the ACM and used by the
Table 2-15 RTC Macro Signal Description
Signal Name
Number of Bits
Direction
Function
RTCMATCH
1
Out
Match between 40-bit counter and match register
RTCPSMMATCH
1
Out
RTCMATCH connected to voltage regulator power
RTCXTLMODE[1:0]
2
Out
Drives XTLOSC RTCMODE[1:0] pins
RTCXTLSEL
1
Out
Drives XTLOSC SELMODE pin
RTCCLK
1
In
RTC clock input from XTLOSC CLKOUT pin
Table 2-16 RTC ACM Memory Map
ACM_ADDR[7:0] Decimal Register Name
Description
Use
0x40
64
COUNTER0
Counter bits 7:0
Used to preload the counter to a
specified start point. Default setting
is all zeroes.
0x41
65
COUNTER1
Counter bits 15:8
0x42
66
COUNTER2
Counter bits 23:16
0x43
67
COUNTER3
Counter bits 31:24
0x44
68
COUNTER4
Counter bits 39:32
0x48
72
MATCHREG0
Match register bits 7:0
The RTC uses a 40-bit register to
compare against the 40-bit counter
value to determine when a match
occurs. This 40-bit match register, like
the counter, is broken into 5 bytes
(MATCHREG0–4).
0x49
73
MATCHREG1
Match register bits 15:8
0x4A
74
MATCHREG2
Match register bits 23:16
0x4B
75
MATCHREG3
Match register bits 31:24
0x4C
76
MATCHREG4
Match register bits 39:32
0x50
80
MATCHBITS0
Individual match bits 7:0
Each bit of the 40-bit counter is
compared to each bit of the 40-bit
match register via XNOR gates. These
40 match bits are partitioned into 5
bytes.
0x51
81
MATCHBITS1
Individual match bits 15:8
0x52
82
MATCHBITS2
Individual match bits 23:16
0x53
83
MATCHBITS3
Individual match bits 31:24
0x54
84
MATCHBITS4
Individual match bits 39:32
0x58
88
CTRL_STAT
Control
(write)
/
Status
(read) register bits 7:0
Control (write) / Status (read) register
bits 7:0
0x59
89
TEST_REG
Test register(s)
Note: Accessing RTC Registers: When reading the RTC count or match register, which operates in the XTLCLK
domain, the appropriate 40-bit value is first copied to a capture register through clock synchronization
circuitry, if and only if the least significant byte of that set of register is addressed. Higher-order bytes of
the same set of registers captured with the LSB can then be read on immediately later read cycles. Higher-
order bytes of that set of registers can be read in any order but must be read before switching to a
different set of registers to ensure data consistency. For example, RTC counter address ranges from 0x40
to 0x44, register 0x40 must be accessed first before accessing addresses 0x41, 0x42, 0x43, and 0x44 to get
the full 40-bit value.