Embedded Flash memory interface
RM0090
80/1731
DocID018909 Rev 11
3.5 Read
interface
3.5.1
Relation between CPU clock frequency and Flash memory read time
To correctly read data from Flash memory, the number of wait states (LATENCY) must be
correctly programmed in the Flash access control register (FLASH_ACR) according to the
frequency of the CPU clock (HCLK) and the supply voltage of the device.
The prefetch buffer must be disabled when the supply voltage is below 2.1 V. The
correspondence between wait states and CPU clock frequency is given in
and
Note:
On STM32F405xx/07xx and STM32F415xx/17xx devices:
- when VOS = '0', the maximum value of f
HCLK
= 144 MHz.
- when VOS = '1', the maximum value of f
HCLK
= 168 MHz.
On STM32F42xxx and STM32F43xxx devices:
- when VOS[1:0] = '0x01', the maximum value of f
HCLK
is 120 MHz.
- when VOS[1:0] = '0x10', the maximum value of f
HCLK
is 144 MHz. It can be extended to
168 MHz by activating the over-drive mode.
- when VOS[1:0] = '0x11, the maximum value of f
HCLK
is 168 MHz. It can be extended to
180 MHz by activating the over-drive mode.
- The over-drive mode is not available when V
DD
ranges from 1.8 to 2.1 V.
Section 5.1.4: Voltage regulator for STM32F42xxx and STM32F43xxx
for details on
how to activate the over-drive mode.
Table 10. Number of wait states according to CPU clock (HCLK) frequency
(STM32F405xx/07xx and STM32F415xx/17xx)
Wait states (WS)
(LATENCY)
HCLK (MHz)
Voltage range
2.7 V - 3.6 V
Voltage range
2.4 V - 2.7 V
Voltage range
2.1 V - 2.4 V
Voltage range
1.8 V - 2.1 V
Prefetch OFF
0 WS (1 CPU cycle)
0 < HCLK
≤
30
0 < HCLK
≤
24
0 < HCLK
≤
22
0 < HCLK
≤
20
1 WS (2 CPU cycles)
30 < HCLK
≤
60
24 < HCLK
≤
48
22 < HCLK
≤
44
20 <HCLK
≤
40
2 WS (3 CPU cycles)
60 < HCLK
≤
90
48 < HCLK
≤
72
44 < HCLK
≤
66
40 < HCLK
≤
60
3 WS (4 CPU cycles)
90 < HCLK
≤
120
72 < HCLK
≤
96
66 < HCLK
≤
88
60 < HCLK
≤
80
4 WS (5 CPU cycles)
120 < HCLK
≤
150
96 < HCLK
≤
120
88 < HCLK
≤
110
80 < HCLK
≤
100
5 WS (6 CPU cycles)
150 < HCLK
≤
168
120 < HCLK
≤
144
110 < HCLK
≤
132
100 < HCLK
≤
120
6 WS (7 CPU cycles)
144 < HCLK
≤
168
132 < HCLK
≤
154
120 < HCLK
≤
140
7 WS (8 CPU cycles)
154 < HCLK
≤
168
140 < HCLK
≤
160